THE   INTERPRETATION   OF   RADIUM 


THE  INTERPRETATION 
OF   RADIUM 

BEING  THE  SUBSTANCE  OF  SIX  FREE  POPULAR 

EXPERIMENTAL  LECTURES  DELIVERED  AT  THE 

UNIVERSITY  OF  GLASGOW 

BY  FREDERICK  SODDY,  M.A.,  F.R.S. 

\  \ 

INDEPENDENT    LECTURER    IN    PHYSICAL    CHEMISTRY    AND    RADIOACTIVITY 
IN   THE    UNIVERSITY    OF    GLASGOW 


WITH    ILLUSTRATIONS 


THIRD      EDITION 
REVISED    AND    ENLARGED 


NEW  YORK 
G.  P.  PUTNAM'S  SONS 

1912 


Q.C..7& 


PREFACE 

'  I  ^HE  present-day  interpretation  of  radium,,  that 
-L  it  is  an  element  undergoing  spontaneous  dis- 
integration, was  put  forward  in  a  series  of  joint 
scientific  communications  to  the  Philosophical  Mag- 
azine of  1902  and  1903  by  Professor  Rutherford, 
now  of  Manchester  University,  and  myself.  As 
its  application  is  not  confined  to  the  physical 
sciences,  but  has  a  wide  and  general  bearing  on 
our  whole  outlook  upon  Nature,  I  have  attempted 
in  this  book  a  presentation  of  the  subject  in  non- 
technical language,  so  that  the  ideas  involved,  and 
their  bearing  upon  current  thought,  may  be  within 
the  reach  of  the  lay  reader.  Although  written  in 
non-technical  language,  no  effort  has  been  spared 
to  get  to  the  root  of  the  matter  and  to  secure 
accuracy,  so  that  possibly  the  book  may  prove 
serviceable  to  workers  in  other  fields  of  science 
and  investigation  as  well  as  to  the  general  public. 
The  book  contains  the  main  substance  of  six 


256991 


vi  PREFACE 

popular  experimental  lectures  delivered  in  the  Uni- 
versity of  Glasgow  at  the  beginning  of  the  year, 
but  being  relieved  from  the  necessity,  always  pre- 
sent in  lecturing,  of  co-ordinating  the  experimental 
and  descriptive  sides,  I  have,  while  adhering  to  the 
lecture  form  of  address,  entirely  rearranged  and 
very  largely  rewritten  the  subject  matter  in  order 
to  secure  the  greatest  possible  degree  of  continuity 
of  treatment.  Certain  portions  of  the  lectures,  for 
example  those  dealing  with  the  X-rays  and  the 
spectra  of  elements,  have  been  omitted,  and  atten- 
tion thereby  concentrated  upon  radium,  the  chief 
topic.  In  addition,  I  have  briefly  embodied  the 
results  of  important  discoveries  which  have  ap- 
peared since  the  date  of  the  lectures,  particularly 
the  experiments  of  Professor  Rutherford  and  Dr. 
Geiger  in  counting  the  number  of  a-particles  ex- 
pelled by  radium.  The  book  also  contains  some 
account  of  the  arrangement  by  means  of  which  I 
have  recently  succeeded  in  detecting  and  measuring 
the  quantity  of  the  helium  generated  from  the 
common  radio- elements  uranium  and  thorium. 

I  have  borrowed  freely  from  numerous  scattered 
lectures  and  addresses  bearing  on  the  subject  which 
I  have  from  time  to  time  been  invited  to  deliver, 


PREFACE  vii 

and  may  mention  in  particular  the  Wilde  lecture  to 
the  Manchester  Literary  and  Philosophical  Society, 
1904,  the  Presidential  and  other  addresses  to  the 
Rontgen  Society,  1906,  the  opening  of  the  discus- 
sion on  the  evolution  of  the  elements  in  Section  A 
of  the  British  Association  Meeting  in  York,  1906, 
and  the  Watt  lecture  to  the  Greenock  Philosophical 

Society,  1908. 

FREDERICK  SODDY. 

THE  UNIVERSITY,  GLASGOW, 
Nov  ember )   1908. 


PREFACE  TO  THE  THIRD  EDITION 

IN  revising  this  book  and  bringing  it  up  to  date, 
I  have  made  it  conform  to  what  I  should  have 
said  if  I  had  been  lecturing  this  year  instead  of  four 
years  ago.  The  statements  of  the  original  edition 
stand  with  hardly  any  serious  modification,  and  this 
is,  perhaps,  the  best  possible  answer  to  any  mis- 
givings that  it  aroused,  that  the  time  had  not  yet 
arrived  for  the  full  acceptance  of  the  new  views  on 
atomic  disintegration,  and  the  unreserved  statement 


viii         PREFACE   TO  THE   THIRD  EDITION 

of  all  the  logical  consequences  that  the  science  of 
radioactivity  has  rendered  necessary.  I  have  taken 
the  opportunity  of  including  the  latest  and  most 
complete  data  available,  and  many  new  discoveries 
and  developments,  in  so  far  as,  among  those  who 
have  devoted  themselves  to  the  subject,  there  is 
a  general  consensus  of  opinion  that  the  evidence 
is  good. 

The  original  lectures  dealt  almost  entirely  with 
radium  and  the  other  members  of  the  uranium 
series.  But  the  thorium  series,  since  the  technical 
production  of  the  new  radioactive  substances, 
mesothorium  and  radiothorium,  now  has  a  con- 
siderable and  growing  importance.  A  new  final 
chapter  has  therefore  been  added,  including  this 
series  and  that  of  actinium,  in  the  hope  of  affording 
those  who  wish  to  pursue  the  subject  further  a 
comprehensive  glimpse  of  the  whole  field  of  radio- 
activity. 

F.  S. 

THE  UNIVERSITY,  GLASGOW, 
August,  1912. 


CONTENTS 


CHAPTER  I 

PAGE 

The  new  science  of  radioactivity — Its  discovery — The  four  ex- 
perimental effects  of  radioactivity — The  rays  of  radioactive 
substances — The  continuous  emission  of  energy  from  the 
radio-elements — The  arresting  feature  of  radioactivity  .  .  i 


CHAPTER   II 

Mme  Curie's  discovery  of  radium — Radioactivity  an  atomic 
property — Its  unalterability — The  radioactivity  of  thorium- 
Pitchblende — Quantity  of  radium  in  pitchblende — The  inter- 
national radium  standard — The  smallest  quantity  of  radium 
that  can  be  detected — Experiments  with  pure  radium  bromide 
— Doctrine  of  energy — Energy  evolved  by  radium  compared 
with  that  from  the  burning  of  coal — Source  of  cosmical 
energy — A  quotation  from  Professor  Tait — Radium  and  the 
"physically  impossible  " 16 


CHAPTER  III 

The  radiations  of  the  radio-elements — a-,  /3-,  and  y-rays — Test  of 
penetrating  power — Experiments  with  the  penetrating  /3-  and 
-y-rays — The  non-penetrating  a-rays — Experiment  to  show  the 
absorption  of  a-rays  by  air — Physical  nature  of  radiations — 
Necessity  for  an  ether — Corpuscular  and  wave  radiations — 
a-  and  /3-  rays  due  to  the  expulsion  of  particles — Can  a  single 
a-particle  be  detected? — The  spinthariscope — Counting  the 
a  particles  expelled  by  radium  .......  39 


x  CONTENTS 

CHAPTER   IV 

PAGE 

The  /3  rays— Their  deviability  by  a  magnet— The  nature  of  the 
/3-particle — Analogy  to  cathode-rays  or  "Radiant  Matter" — 
The  electron — Velocity  of  the  /3-rays — The  nature  of  the 
a-particle — Its  velocity — Its  power  of  passing  through  atoms 
of  matter  in  its  path — Scattering  of  a-particles — Method  of 
rendering  the  track  of  rays  visible — a-Particles  might  be 
expelled  without  their  being  detectable  .....  66 


CHAPTER  V 

From  where  does  the  energy  of  radium  come  ? — The  two  alterna- 
tives and  their  consequences — The  internal  energy  of  matter 
—  Atomic  disintegration  —  Disintegration  in  cascade — The 
successive  outbursts  of  energy  from  otherwise  impalpable 
quantities  of  matter — The  emanation  of  radium — Its  proper- 
ties— Experiments  with  the  emanation — Its  condensation  by 
liquid  air — The  infinitesimal  quantity  of  the  emanation  from 
radium — The  chemical  nature  and  atomic  weight  of  the 
emanation — The  energy  evolved  by  the  emanation — The 
decay  of  the  emanation  and  its  reproduction  by  radium — The 
facts,  not  the  theories,  of  radioactivity  are  revolutionary — 
Unalterability  of  radioactive  changes — Evanescent  products 
of  radioactive  change — All  products  equally  knowable  whether 
short-lived  or  long 93 


CHAPTER  VI 

The  connection  of  the  a-particle  with  radioactive  changes — The 
a-particle  and  helium — Accumulation  of  helium  in  geological 
time — Discovery  of  helium  in  the  sun  and  on  the  earth — Its 
connection  with  radioactivity — Production  of  helium  from 
radium — Its  production  from  uranium  and  thorium — Proof 
that  the  a-particle  is  an  atom  of  helium — The  nature  of  the 
first  change  of  radium— Radioactive  "  recoil"  .  .  .  .128 


CONTENTS  xi 


CHAPTER   VII 

Atomic  disintegration  and  the  periodic  law — Questions  of  nomen- 
clature— Definition  of  the  chemist's  atom— Difference  between 
atoms  and  chemical  compounds — The  insufficiency  of  chemical 
methods  in  many  radioactive  problems — The  relation  of  radio- 
activity to  the  electrical  theory  of  matter — Hypotheses  or 
mental  pictures — The  two  possible  pictures  of  atomic  dis- 
integration— Sudden  explosive  character  of  the  disintegration 
— Law  of  radioactive  changes— Chance  of  disintegration — 
Average  life  of  a  disintegrating  atom — Its  expectation  of  life 
—  The  "how,"  not  the  "why,"  of  atomic  disintegration  ex- 
plained— Determination  of  the  period  of  average  life  of  atoms 
— Primary  radio-elements  and  ephemeral  transition-forms — 
Radioactive  equilibrium — Average  life  of  radium — The  total 
energy  evolved  in  the  complete  disintegration  of  radium  .  .  145 


CHAPTER  VIII 

How  is  it  there  is  any  radium  left? — The  parent  of  radium — 
Fixity  of  ratio  between  the  quantity  of  uranium  and  radium 
in  all  minerals — Period  of  average  life  of  uranium — Relation 
of  uranium  to  radium — An  analogy  to  the  Glasgow  water- 
supply  system — Age  of  pitchblende — Radioactivity  of  uranium 
— Uranium  X,  Uranium  I.,  and  Uranium  II. — Uranium  not 
the  direct  parent  of  radium — Growth  of  radium  by  uranium — 
Intermediate  transition  forms  of  long  life — Ionium,  the  direct 
parent  of  radium  —  The  stately  procession  of  elementary 
evolution  ...........  168 


CHAPTER  IX 

The  subsequent  changes  of  radium — The  induced  or  excited 
radioactivity — The  active  deposit  of  radium — The  disintegra- 
tion of  the  emanation — Radium  A,  B,  C — Experiments  with 
the  active  deposit — Radium  A  gives  only  a-rays  and  has  a 
very  short  life — Radium  B  gives  no  rays — Radium  C  gives 


xii  CONTENTS 

PAGK 

a-,  /3-,  and  y-rays — The  emanation  only  gives  a-rays — The 
later  slow  changes  of  radium  —  Radium  D,  E,  and  F  — 
Polonium — Its  identity  with  radium  F — The  last  disintegra- 
tion— What  is  the  ultimate  product  ? 190 

CHAPTER  X 

Ratio  of  quantities  of  polonium  and  radium  in  minerals — Table 
of  the  ratio  of  the  quantities  of  all  the  products  of  uranium — 
Difficulty  of  concentrating  many  of  the  long-lived  products 
of  uranium — Increase  of  activity  of  radium  with  time — 
Radioactivity  a  physical  measure  of  value  or  rarity — The 
currency  metals  and  their  rarity— Are  they  changing  like 
radium  ? — Physical  necessity  for  rarity  of  a  changing  element 
— One  aspect  of  the  ultimate  nature  of  matter — A  quotation 
from  Clerk  Maxwell— Evolution  of  the  elements  denied — 
Similarity  of  all  the  atoms  of  the  same  element — The  atom 
a  complex  and  perfect  piece  of  mechanism  —  Professor 
Schuster's  analogy — The  atom  true  to  its  character  at  dissolu- 
tion— Similarity  in  the  velocity  of  all  a-particles  expelled  from 
a  radio-element  —  The  more  stable  the  radio-element  the 
slower  the  a-rays  expelled  from  it — Survival  of  the  fittest  or 
most  stable  atoms — Universality  of  the  conception  of  evolution 
to  the  material  universe,  animate  and  inanimate  .  .  .210 

CHAPTER  XI 

Why  is  radium  unique  among  the  elements? — Its  rate  of  change 
only  makes  it  remarkable — Uranium  is  more  wonderful  than 
radium — The  energy  stored  up  in  a  pound  of  uranium — 
Transmutation  is  the  key  to  the  internal  energy  of  matter — 
The  futility  of  ancient  alchemy — The  consequences  if  trans- 
mutation were  possible — Primitive  man  and  the  art  of  kind- 
ling fire — Modern  man  and  the  problem  of  transmutation — 
Cosmical  evolution  and  atomic  disintegration — Radioactivity 
and  geology — Quantity  of  radium  in  the  earth's  crust — The 
earth  probably  not  a  cooling  globe — The  geological  age  and 
the  incandescent  age — Ancient  mythology  and  radioactivity — 
The  serpent  "  Ouroboros  " — The  "  Philosopher's  Stone  "  and 


CONTENTS  xiii 

PAGE 

the  "  Elixir  of  Life  "—The  "  Fall  of  Man  "  and  the  "  Ascent  of 
Man  " — The  great  extension  in  the  possible  duration  of  past 
time — Radium  and  the  struggle  for  existence — Existence  as  a 
struggle  for  physical  energy — The  new  prospect  .  .  .  230 


CHAPTER    XII 

The  thorium  disintegration  series — Source  and  radioactivity  of 
thorium  compounds — Mesothorium  and  radiothorium — Varia- 
tion of  radioactivity  of  thorium  compounds  with  time  — 
Analogy  between  the  thorium  and  uranium  series — Meso- 
thorium and  its  changes  of  activity  with  time — Its  chemical 
resemblance  to  radium — Abundance  of  the  raw  material  of 
mesothorium  —  The  thorium  emanation  —  Experiments  with 
radiothorium — Thorium  A — The  actinium  disintegration  series 
—The  question  of  the  origin  of  actinium — Its  great  scarcity — 
Multiple  atomic  disintegration — The  actinium  emanation — 
Actinium  A — The  unsolved  riddle  of  matter  ....  256 

INDEX „  279 


LIST   OF    ILLUSTRATIONS 


1.  Becquerel's  uranium  radiograph  of  an  aluminium  ^ 

medallion        .  .  .  '         '  I   TV?  face       10 

2.  Welsbach   mantle,  taken  by  the  rays   from   the  f 

thorium  contained  in  it  .  .         J 

3.  Photograph  and  radiograph  of  a  piece  of  pitch- 

blende (Sir  William  Crookes)       .  „  20 

4.  Photograph  of  silk  tassel  electrified  by  friction     .) 

5.  The  same  discharged  by  the  rays  of  radium          .J 

6.  Radium  writing  on  a  photographic  plate        .         .^ 

7.  Box  of  compasses  taken  by  7-rays  of  radium        ./ 

8.  Diagram  of  coated  flask  and  radium-covered  dish 

for  showing  a-rays          .  .  ....       49 

9.  Photograph  of  the  same  apparatus    .  .        .      To  face      49 

10.  Diagram     of     Spinthariscope    of     Sir     William 

Crookes  .  .  .  ....       60 

11.  Photograph  of  the  Spinthariscope      ,  .        .       To  face      49 

12.  Photograph  of  the  electro-magnet  for  deviating 

the  £-rays        .  .  .  .  „  66 

13.  Diagram  of  magnetic  deviation  of  /3-rays      ....      68 

14.  Diagram  of  Crookes'  tube  to  show  magnetic  devia- 

tion of  cathode-rays       .  .  75 

15.  Diagram  of  Strutt's  radium  clock       .  82 

16.  Photograph  of  radium  clock  .  .        .       To  face      66 

17.  Photograph  of  tube  containing  willemite       .         .] 

1 8.  Photograph  of  the  same  tube  by  its  own  light  when  \        „          108 

containing  radium  emanation       .  J 

19.  Diagram  of  apparatus  for  showing  the  condensa- 

tion of  the  radium  emanation       .  .         .         .         .112 

20.  Diagram  of  the  first  disintegration  of  radium        .         .         .129 


XVI 


LIST  OF  ILLUSTRATIONS 


21.  Photograph  of  the  spectrum-tube  in  which  the  pro-^ 

duction  of  helium  from  radium  emanation  was 

observed  .  .  .  .        .  J.  To  fact    137 

22.  Photograph   by   Dr.    Giesel   of    the   spectrum   of 

helium  produced  from  radium 

23.  Photograph  of  apparatus  for  detecting  and  measur- 

ing helium  produced  from  uranium  and  thor- 
ium  .  .  .        .  „          138 

24.  Diagram  showing  the  first  change  of  radium          .         .         .     142 

25.  Diagram  for  the  first  disintegration  of  uranium      .         .         .181 

26.  Diagram   of   the    uranium-radium    disintegration 

series  (initial  changes)    .  .  188 

27.  Diagram  of  the  first  four  disintegrations  of  radium         .         .     194 

28.  Diagram  of  apparatus   for  obtaining   the   active 

deposit  of  radium  .  .  .... 

29.  Photograph  of  the  same  apparatus      .          -. .        .      To  face 

30.  Diagram  of  the  later  disintegrations  of  radium 

31.  Diagram  of  the  complete  uranium  disintegration 

series  .  .  ,  .         *         .         . 

32.  Diagram  of  the  complete  thorium  disintegration 

series  .  .  ..... 

33.  Diagram  of  the  complete  actinium  disintegration 

series 


196 
196 
204 

209 
262 
273 


THE    INTERPRETATION 
OF  RADIUM 

CHAPTER   I. 

The  new  science  of  radioactivity — Its  discovery — The  four  experi- 
mental effects  of  radioactivity — The  rays  of  radioactive  substances 
— The  continuous  emission  of  energy  from  the  radio-elements — 
The  arresting  feature  of  radioactivity. 

ONE  of  the  main  duties  of  science  is  the  corre- 
lation of  phenomena,  apparently  disconnected 
and  even  contradictory.  For  example,  chemistry 
teaches  us  to  regard  under  one  aspect,  as  various 
types  of  combustion  or  oxidation,  the  burning  of 
a  candle,  the  rusting  of  metals,  the  physiological 
process  of  respiration,  and  the  explosion  of  gun- 
powder. In  each  process  there  is  the  one  common 
fact  that  oxygen  enters  into  new  chemical  com- 
binations. Similarly  to  the  physicist,  the  fall  of 
the  traditional  apple  of  Newton,  the  revolution 
of  the  earth  and  planets  round  the  sun,  the  appari- 
tions of  comets,  and  the  ebb  and  flow  of  the  tides 


2  THE  NE  W  SCIENCE 

are  all  phases  of  the  universal  law  of  gravitation 
A  race  ignorant  of  the  nature  of  combustion  or  of 
the  law  of  gravitation,  and  ignorant  of  the  need 
of  such  generalisations,  could  not  be  considered  to 
have  advanced  far  along  the  paths  of  scientific 
discovery.  The  phenomena  with  which  I  am  con- 
cerned in  these  lectures  belong  to  the  newly-born 
science  of  radioactivity  and  to  the  spontaneous 
disintegration  of  elements  which  the  study  of  radio- 
activity has  revealed  to  us.  It  is  a  natural  inquiry 
to  ask — To  what  most  nearly  are  these  new  pheno- 
mena correlated  ?  Is  it  possible  to  give,  by  the 
help  of  an  analogy  to  familiar  phenomena,  any 
correct  idea  of  the  nature  of  this  new  phenomenon 
"  Radioactivity  "  ?  The  answer  may  surprise  those 
who  hold  to  the  adage  that  there  is  nothing  new 
under  the  sun.  Frankly,  it  is  not  possible,  because 
in  these  latest  developments  science  has  broken 
fundamentally  new  ground,  and  has  delved  one 
distinct  step  further  down  into  the  foundations  of 
knowledge. 

During  the  century  which  has  just  closed  there 
occurred,  it  is  true,  at  an  ever-increasing  rate,  a 
ceaseless  extension  of  our  knowledge  of  the  nature 
of  matter  upon  which  physical  science  is  largely 
based.  Yet  this  advance  was  for  the  most  part  an 
expansion  rather  than  a  deepening.  It  was  con- 
cerned with  what  may  be  termed  atomic  and 


THE  NE  W  SCIENCE  3 

molecular  architecture,  the  external  qualities  of 
atoms  and  the  construction  and  study  of  com- 
plexes built  of  atoms — that  is  to  say,  molecules. 
As  buildings  are  built  of  bricks,  so  compounds  can 
nowadays  be  built  up  out  of  atoms.  The  atoms 
are  to  the  chemist  and  physicist  what  bricks  are 
to  the  architect — the  units  supplied  ready-made  to 
a  certain  limited  number  of  standard  specifications 
and  dimensions  capable  of  an  endless  variety  of 
combinations  and  arrangements,  each  with  its  own 
peculiarities  and  external  relationships. 

The  century  which  has  just  begun  has  seen  the 
first  definite  and  considerable  step  taken  into  the 
ultimate  nature  of  these  units  of  matter  or  atoms, 
which  is  in  one  sense  not  merely  an  extension  of 
existing  knowledge  or  principles,  but  a  radically 
new  departure.  Radioactivity  is  a  new  primary 
science  owing  allegiance  neither  to  physics  nor 
chemistry,  as  these  sciences  were  understood  before 
its  advent,  because  it  is  concerned  with  a  know- 
ledge of  the  elementary  atoms  themselves  of  a 
character  so  fundamental  and  intimate  that  the  old 
laws  of  physics  and  chemistry,  concerned  almost 
wholly  with  external  relationships,  do  not  suffice. 
This  first  step  has  indeed  emphasised  how  super- 
ficial our  knowledge  of  matter  has  really  been. 
If  one  were  to  demonstrate  to  an  architect  that 
the  bricks  he  habitually  and  properly  employs  in 


4  THE  NE  W  SCIENCE 

his  constructions  were  under  other  circumstances 
capable  of  entirely  different  uses — let  us  say,  for 
illustration,  that  they  could  with  effect  be  employed 
as  an  explosive  incomparably  more  powerful  in  its 
activities  than  dynamite — the  surprise  of  the  archi- 
tect would  be  no  greater  than  the  surprise  of  the 
chemist  at  the  new  and  undreamt  of  possibilities  of 
matter  demonstrated  by  the  mere  existence  of  such 
an  element  as  radium.  ^ 

In  this  first  lecture  our  attention  will  be  mainly 
directed  to  the  one  outstanding  feature  in  connec- 
tion with  radium,  and  the  property  of  radioactivity 
which  it  exhibits  to  an  extraordinary  degree,  in 
which  the  whole  range  of  its  remarkable  features 
are  epitomised.  \The  radioactive  substances  evolve 
a  perennial  supply  of  energy  from  year  to  year 
without  stimulus  and  without  exhaustion.  It  would 
be  idle  to  deny  with  regard  to  this  that  physical 
science  was  taken  completely  by  surprise.  Had 
any  one  twelve  years  ago  ventured  to  predict 
radium  he  would  have  been  told  simply  that  such 
a  thing  was  not  only  wildly  improbable,  but  actually 
opposed  to  all  the  established  principles  of  the 
science  of  matter  and  energy.  So  drastic  an 
innovation  was,  it  is  true,  unanticipated.  Radium, 
however,  is  an  undisputed  fact  to-day,  and  there  is 
no  question  which  would  have  triumphed  in  the 


THE  NEW  SCIENCE  5 

conflict,  had  its  existence  conflicted  with  the  estab- 
lished principles  of  science.  Natural  conservatism 
and  dislike  of  innovation  appear  in  the  ranks  of 
science  more  strongly  than  most  people  are  aware. 
Indeed,  science  is  no  exception.  There  was, 
however,  never  the  slightest  ground  for  assuming 
that  because  the  new  facts  were  startling  and  un- 
expected they  must  necessarily  conflict  with  older 
knowledge.  That  would  be  to  pay  science  a  poor 
compliment.  Some  of  the  new  facts  we  shall 
discuss  in  the  lectures  appeared  at  first,  and  may 
even  yet  appear  to  you,  almost  incredible,  but  that 
is  only  on  account  of  the  entire  newness  of  the 
whole  region  to  which  they  belong.  Into  this 
region  the  older  chemistry  and  physics  have,  as  we 
have  seen,  never  before  penetrated.  It  is  not 
until  we  begin  to  apply  to  the  new  facts  the  estab- 
lished principles  of  science,  which  have  served  so 
well  of  old,  that  their  full  significance  gradually 
becomes  evident.  Keep  in  mind  that  our  know- 
ledge of  nature  is  always  of  necessity  partial,  and 
is  bounded  in  all  directions  by  certain  inevitable 
but  too  often  forgotten  limitations  connected,  for 
example,  with  the  briefness  of  human  life  and  the 
physical  impossibility  of  pursuing  investigations 
except  under  conditions  where  the  life  of  the  in- 
vestigator can  be  maintained.  The  laws  and  prin- 


6  THE  NEW  SCIENCE 

ciples  of  physical  science,  old  and  new,  are  alike 
subject  to  these  perpetual  limitations,  and  are 
necessarily  only  true  within  these  limits.  From 
this  point  of  view  there  is  nothing  in  the  many 
surprising  properties  of  radium  which  conflicts  with 
a  single  established  principle  of  older  science. 
Physics  and  chemistry  remain  almost  unchanged 
where  they  were,  and  radioactivity,  so  far  as  it  is 
concerned  with  the  correctness  of  their  principles, 
has,  as  a  matter  of  fact,  given  to  the  old  laws  and 
theories  a  fuller  and  truer  significance  than  they 
had  before.  The  extension  of  the  old  theories 
which  has  been  rendered  necessary  has  not  been 
revolutionary  in  any  destructive  sense.  It  is 
wonderful  how  accommodating  a  true  theory  is  to 
new  truth,  apparently  of  a  diametrically  opposite 
character,  and  this  not  in  any  sense  of  mere 
ingenuity  of  explanation,  but  in  a  manner  that 
arrests  the  investigator,  and  is  his  sign  that  he  is 
on  safe  ground.  It  may  seem  a  paradox,  but  from 
the  first  the  best  proof  of  the  newer  views,  to  my 
mind,  was  in  the  completeness  with  which  the 
strange,  newly-won  knowledge  of  radioactivity 
harmonised  with  the  old  views  of  the  chemist 
about  atoms  and  elements.  On  the  other  hand 
this  gratifying  harmony,  where  conflict  might  have 
been  expected,  is  not  a  surrender.  On  every  hand 
new  vistas  of  thought  are  opening  out.  We  see 


THE  NEW  SCIENCE  7 

the  simple  and  direct  answer  to  many  problems 
before  deemed  ,  insoluble.  We  recognise  now 
causes  at  work  where  before  we  only  saw  effects, 
many  of  them  so  familiar  and  ingrained  in  our 
consciousness  that  the  necessity  for  a  cause  had 
been  almost  overlooked,  or,  if  felt  at  all,  met  per- 
functorily and  wholly  inadequately  by  existing 
knowledge.  Highly  technical  and  complicated  as 
many  of  the  researches  on  radioactivity  are,  the 
main  conclusions  of  the  science  are  as  simple 
and  certain  as  they  are  fundamental,  and  of 
general  interest.  It  is  the  duty  of  every  educated 
man  to  make  himself  aware  of  the  chief  bearings 
of  these  conclusions,  for  they  touch  human  life 
strangely  at  many  points,  and  are  destined  in  the 
future  to  influence  profoundly  the  course  of  philo- 
sophic thought.  In  a  few  years  the  elementary 
principles  of  radioactivity  will  be  taught  in  all 
schools  as  belonging  to  the  very  beginnings  of 
physical  science.  To-night,  while  all  is  strange 
and  new  and  the  very  name  of  the  science  even 
unfamiliar,  it  may  appear  a  far  cry  to  attempt  to 
foretell  the  effects  these  discoveries,  concerned 
primarily  with  the  ultimate  nature  of  matter,  are 
destined  to  exert  on  our  conceptions  of  the  ultimate 
destiny  of  man.  But  already  the  most  direct  con- 
nection is  apparent.  Indeed,  this  aspect  of  the 
advance  is  perhaps  the  most  revolutionary.  We 


8  DISCOVERY  OF  RADIOACTIVITY 

shall  be  able  to  see  more  clearly  at  the  end  how 
this  has  come  about.  At  present  it  is  sufficient  to 
indicate-  that  radioactivity  has  introduced  a  new 
conception  into  the  fundamental  problems  of  ex- 
istence. By  its  conclusion  that  there  is  imprisoned 
in  ordinary  common  matter  vast  stores  of  energy, 
which  ignorance  alone  at  the  present  time  prevents 
us  from  using  for  the  purposes  of  life,  radioactivity 
has  raised  an  issue  which  it  is  safe  to  say  will  mark 
an  epoch  in  the  progress  of  thought.  With  all  our 
mastery  over  the  powers  of  Nature  we  have 
adhered  to  the  view  that  the  struggle  for  existence 
is  a  permanent  and  necessary  condition  of  life. 
To-day  it  appears  as  though  it  may  well  be  but 
a  passing  phase,  to  be  altogether  abolished  in  the 
future  as  it  has  to  some  extent  been  mitigated  in 
the  past  by  the  unceasing,  and  as  it  now  appears, 
unlimited  ascent  of  man  to  knowledge,  and  through 
knowledge  to  physical  power  and  dominion  over 
Nature. 

The  first  discovery  of  the  property  we  now  call 
radioactivity  was  made  in  the  year  1896  by  M. 
Henri  Becquerel  in  Paris,  and,  like  many  other 
great  discoveries,  the  actual  experiment  itself  owed 
something  to  luck  or  chance  or  accident.  Looking 
backward,  however,  it  appears  rather  that  only  the 
particular  day  or  month  of  the  discovery  was  a 


DISCOVERY  OF  RADIOACTIVITY  9 

matter  of  chance.  The  time  was  just  ripe  for  the 
event,  and  it  is  certain  that  its  coming  could  not 
long  have  been  delayed.  Some  slight  historical 
sketch  of  the  conditions  preceding  and  immediately 
following  the  discovery  is  necessary  before  consider- 
ing wherein  lies  its  great  significance.  The  memor- 
able discovery  of  the  X-rays  by  Professor  Rontgen, 
in  1895,  which  is  known  to  all,  familiarised  scientific 
workers  with  a  type  of  radiation  able  to  traverse 
objects  opaque  to  light.  The  X-rays  are  them- 
selves invisible  to  the  unaided  eye,  but  are  able  to 
affect  the  photographic  plate.  This  led  to  experi- 
ments being  made  in  order  to  see  if  similar  types  of 
rays  were  not  produced  in  other  ways,  As  you  all 
know,  certain  substances  exposed  to  sunlight  shine 
afterwards  in  the  dark,  and  this  property,  which  finds 
an  application  in  the  manufacture  of  luminous  paint, 
is  known  as  phosphorescence  or  fluorescence.  Is 
phosphorescent  light  entirely  stopped  by  opaque 
objects  ?  Or  does  it  in  part  consist  of  invisible 
penetrating  rays  like  the  X-rays?  M.  Becquerel 
wrapped  a  photographic  plate  in  black  paper  and 
placed  on  it  a  phosphorescent  substance  which  was 
then  exposed  to  sunlight.  By  great  good  fortune 
M.  Becquerel  chose  as  the  particular  phosphorescent 
body  a  preparation  of  uranium,  and  found  as  the 
result  of  the  experiment  that  the  photographic 
plate  beneath  the  preparation  was  darkened.  The 


io  DISCOVERY  OF  RADIOACTIVITY 

preparation  had  given  out  rays  which,  unlike  sun- 
light, were  capable  of  penetrating  the  black  paper. 
It  was  soon  found  that  these  rays,  like  the  X-rays, 
even  penetrated  thin  plates  of  metal,  for  when  such 
a  thin  plate  was  interposed  between  the  preparation 
and  the  film  ^darken ing  still  occurred.  But  one  day, 
the  sun  being  obscured,  the  plate  and  the  phos- 
phorescent uranium  preparation  upon  it  were  set 
aside  in  a  dark  drawer  for  some  weeks,  and  M.  Bec- 
querel,  wishing  to  see  if  any  darkening  had  occurred 
without  the  sunlight,  developed  the  plate  as  it  was. 
It  was  found  that  darkening  had  gone  on  just  as 
much  in  the  darkness  as  in  the  light.  Further 
experiments  soon  established  that  neither  sunlight 
nor  phosphorescence  had  anything  to  do  with  the 
experiment.  The  action  is  an  entirely  new  inherent 
property  of  the  element  uranium.  No  other  phos- 
phorescent body  would  have  darkened  the  plate 
even  in  the  sunlight,  while  all  preparations  contain- 
ing uranium  do  so,  whether  they  are  phos- 
phorescent or  not,  in  total  darkness  as  well  as  in 
the  light.  Fig.  i  shows  one  of  the  photographs  by 
uranium  rays  obtained  by  M.  Becquerel.  Between 
the  patch  of  the  uranium  preparation  and  the  plate 
was  placed  an  aluminium  medallion,  stamped  with  a 
head  of  a  figure  in  relief,  which  partially  shielded 
the  plate  beneath  from  the  rays.  The  impression 
under  the  thinner  portions  of  the  medallion  is 


FIG.  i.     BecquerePs  Uranium  Picture. 


^m^^^^^^^^^^^^^^^^^^^^^^^^^jjjgjjjjjilmf^ 


FIG.  2,     Welsbach  Mantle  imprinted  by  its  own  rays. 


To  face  p.  10. 


EFFECTS   OF  RADIOACTIVITY  n 

darker  than  under  the  thicker  portions,  thus  causing 
the  head  of  the  figure  to  be  clearly  apparent  in  the 
photograph. 

Although  the  radioactive  process  is  itself  without 
analogy  in  science,  the  main  effects  which  it 
produces  can  almost  all  be  more  or  less  nearly 
imitated,  and  were  all  more  or  less  perfectly  studied 
prior  to  its  discovery.  The  main  effects  of  radio- 
activity with  which  we  are  most  concerned  are 
four.  Firstly  then,  radioactive  substances  affect  a 
photographic  plate  in  the  same  manner  as  light  and 
many  other  agencies.  Secondly,  they  excite  phos- 
phorescence or  fluorescence  in  certain  substances 
when  brought  in  their  neighbourhood.  Thirdly, 
radioactive  bodies  cause  the  air  and  other  gases  to 
lose  the  insulating  power  they  normally  possess 
and  to  become  partial  conductors  of  electricity.  In 
consequence,  any  electrified  object  has  its  elec- 
tricity rapidly  discharged  in  the  neighbourhood  of 
a  radioactive  substance.  But  the  same  effect  is 
produced  by  X-rays,  by  incandescent  bodies,  and 
even  by  a  lighted  match.  The  instrument  em- 
ployed to  detect  this  effect  is  the  gold-leaf  electro- 
scope, the  first  and  simplest  electrical  instrument  to 
be  invented,  and  for  this  purpose  capable  of  so 
great  refinement  that  it  affords  the  most  delicate 
and  sensitive  test  it  is  possible  to  employ  in 


12        XAYS   OF  RADIOACTIVE   SUBSTANCES 

the  detection  of  radioactivity.  Lastly,  radioactive 
bodies  generate  heat,  as  does  coal  or  any  other 
substance  burning.  The  photographic  action  and 
the  discharge  of  electricity  from  insulated  charged 
bodies  are  clearly  shown  by  radioactive  substances 
even  in  the  form  in  which  they  occur  in  Nature,  as 
all  unsuspected  they  have  been  handled  and  ex- 
amined by  men  for  centuries.  Hence  you  will 
understand  how  it  is  that  the  discovery  of  radio- 
activity could  not  under  any  circumstances  have 
been  indefinitely  delayed.  But  only  the  more  power- 
fully radioactive  substances,  like  radium,  give  ap- 
preciable phosphorescence  or  heat  effects.  In  the 
naturally  occurring  radioactive  substances  these 
effects  are  far  too  small  to  be  readily  detectable. 

Exact  physical  experiments  have  demonstrated 
that  all  these  effects  of  radioactivity  owe  their 
origin  to  the  fact  that  the  radioactive  substances 
emit  "rays."  These  rays  are  invisible  to  the  un- 
aided eye  it  is  true.  In  this  they  resemble 
Rontgen's  X-rays.  There  are  three  different  types 
of  rays  given  out  by  the  radioactive  substances, 
which  are  known  respectively  as  the  a-,  ,#-,  and 
y-rays.  Each  will  require  detailed  future  considera- 
tion. But  they  all  bear  less  resemblance  to  light 
than  to  the  recently  discovered  types  of  rays,  of 
which  the  X-rays  of  Rontgen  are  typical,  produced 


RAYS   OF  RADIOACTIVE   SUBSTANCES        13 

when  an  electric  current  is  forced  by  powerful  appli-' 
ances  to  traverse  a  nearly  vacuous  space,  a  path 
which  it  much  prefers  not  to  take  if  it  can  avoid  it. 
The  first  effects  of  most  new  things  are  old. 
Motor-cars  and  railways  do  the  old  work  of  horses. 
In  commercial  life  a  really  new  effect  is  generally 
valueless  until  it  has  ceased  to  be  new,  as  many 
inventors  know  to  their  cost.  In  scientific  dis- 
covery a  new  effect  does  not  usually  proclaim  itself 
from  the  housetops.  It  often  needs  new  instru- 
ments and  the  way  must  first  be  paved  for  its  dis- 
covery, while  old  effects  are  generally  recognised 
first.  It  is  natural  that  the  first  effects  of  radio- 
activity to  be  discovered  should  be  those  more  or 
less  familiar,  But  for  the  development  to  perfec- 
tion of  that  marvellous  thing,  the  photographic 
plate,  radioactivity  would  not  have  been  discovered 
in  the  way  it  was,  and  we  should  still  be  without 
one  of  the  readiest  methods  of  detecting  it.  But 
for  the  work  on  the  conduction  of  electricity 
through  gases  immediately  following  the  discovery 
of  the  X-rays,  the  only  other  method  of  detecting 
radioactivity  in  the  natural  state  would  be  unknown, 
and  therefore  also  in  all  probability  radioactivity 
itself.  On  the  other  hand,  if  radioactive  substances 
exhibit  any  entirely  new  kind  of  properties — and  it 
is  quite  possible  that  they  do — it  is  very  likely  that 
their  very  novelty  would  delay  their  discovery. 


14     ARRESTING  FEATURE  OF  RADIOACTIVITY 

Why  then,  you  may  ask,  if  all  of  the  effects  of 
radioactivity  are  shown  in  other  ways  do  I  insist 
that  radioactivity  is  a  phenomenon  unparalleled  in 
science  ?  The  distinctive  feature  of  radioactivity  is 
not,  however,  so  much  in  the  rays  the  radioactive 
substances  emit,  though  we  shall  find  upon  a  closer 
examination  that  these  are  distinctive  and  most 
remarkable.  The  main  interest  of  the  new  pro- 
perty consists  in  the  spontaneous  and  continuous 
emission  of  energy  of  which  the  rays  are  but  one 
manifestation.  \  Heat  and  light  may  be  obtained  in 
numerous  ways,  but  it  is  a  new  thing  to  find  it  being 
given  out  by  a  substance,  as  it  is  by  radium,  year 
in,  year  out,  without  apparent  intermission  or  dimi- 
nution, and  without  the  substance  being  in  any 
apparent  way  consumed  or  altered.  This  was  the 
arresting  fact.  The  radioactive  substances  appar- 
ently were  performing  the  scientifically  impossible 
feat  of  evolving  a  store  of  energy  presumably  out 
of  nothing.  N»  So  long  as  radioactivity  was  known 
only  on  the  scale  and  in  the  degree  exhibited  by 
uranium,  it  was  perhaps  possible  to  explain  away 
this  aspect  of  the  question  because  of  the  minute- 
ness of  the  amount  of  energy  involved  and  the 
difficulty  of  proving  that  it  was  not  in  some  way 
derived  from  the  surroundings.  But  the  work  of 
M.  and  Mme.  Curie,  by  their  discovery  of  radium, 
made  the  world  familiar  with  an  element  over  a 


ARRESTING  FEATURE  OF  RADIOACTIVITY    15 

million  times  as  radioactive  as  uranium.  In  this 
case  the  energy  evolved  is  great  enough  to  produce 
effects  which  are  obvious  to  all  and  which  cannot  be 
explained  away.  In  a  strictly  scientific  sense  there 
is  no  difference  of  principle  between  the  radio- 
activity of  radium  and  that  of  uranium.  The 
difference  is  one  of  degree  only,  but  it  is  so  great 
that  radium,  though,  as  we  shall  come  to  see,  not  so 
wonderful  in  reality  as  uranium,  rapidly  acquired  a 
monopoly  of  public  interest  and  attention. 


CHAPTER  II. 

Mme.  Curie's  discovery  of  radium — Radioactivity  an  atomic  property 
— Its  unalterability — The  radioactivity  of  thorium— Pitchblende — 
Quantity  of  radium  in  pitchblende — The  International  Radium 
Standard — The  smallest  quantity  of  radium  that  can  be  detected 
— Experiments  with  pure  radium  bromide — Doctrine  of  energy — 
Energy  evolved  by  radium  compared  with-  that  from  the  burning 
of  coal — Source  of  cosmical  energy — A  quotation  from  Professor 
Tail — Radium  and  the  "physically  impossible." 

IT  is  worth  while  to  stop  to  consider  the 
starting-point  of  Mme.  Curie's  discovery. 
Chemistry  analyses  all  known  substances  into  their 
component  constituents  or  elements,  all  of  which 
are  fundamentally  different,  the  one  from  the  other, 
and  inconvertible  the  one  into  the  other.  Uranium 
is  such  an  element,  gold,  silver,  lead,  and  many  of 
the  common  metals  are  others,  but  uranium  is  dis- 
tinguished by  having  relatively  the  heaviest  of  all 
known  atoms.  The  atom  is  the  minimum  unit 
quantity  of  an  element.  The  relative  atomic 
weight  of  an  element  is  one  of  its  most  important 
characteristics,  and  as  a  first  approximation  the 
atom  of  hydrogen  is  chosen  as  the  standard  and  is 
assigned  unit  value.  For  exact  work  it  is  more 
convenient  to  choose  oxygen  as  the  standard,  with 

16 


MME.  CURIE'S  FIRST  STEP  17 

the  value  16.  On  this  basis  the  atomic  weight  of 
hydrogen  becomes  roo8,  and  that  of  uranium  238-5. 
Now  radioactivity  is  an  intrinsic  property  of  the 
element  uranium,  and  therefore  of  the  atom  of 
uranium.  This  Mme.  Curie  first  recognised,  and 
it  formed  the  starting-point  of  her  work.  In  the 
case  of  uranium,  the  element  itself  and  all  its  various 
compounds  are  radioactive,  and  the  radioactivity  of 
each  compound  is  conditioned  simply  by  the  relative 
amount  of  uranium  it  contains.  It  does  not  matter 
where  the  uranium  comes  from— it  is  always  to  the 
same  degree  radioactive.  Non-radioactive  uranium 
is  unknown.  Not  only  so,  but  it  is  absolutely  im- 
possible really  to  affect  the  radioactivity  of  uranium 
or  any  other  of  the  radioactive  elements  in  the 
slightest  degree.  In  this  the  process  is  utterly 
unlike  any  other  process  previously  known  in 
Nature.  Radioactivity  is  part  and  parcel  of  the 
very  nature  of  the  element  which  possesses  the 
property,  and  therefore  of  the  atom  or  unit  quantity 
of  the  element.  The  attempts  that  have  been  made 
artificially  to  alter  or  to  stop  the  radioactivity  of 
an  element  have  met  with  signal  failure.  This  is 
still  an  impossible  feat — a  thing  modern  science 
cannot  do — and  yet,  as  we  shall  come  to  see  quite 
clearly  in  the  sequel,  a  thing  which  science  must  do 
if  mankind  is  to  realise  to  the  full  the  destiny  these 
discoveries  have  for  the  first  time  unveiled.  There 


1 8  THORIUM 

is  another  still  impossible  feat,  to  the  accomplish- 
ment of  which  all  the  appliances  of  modern  science 
have  been  directed  in  vain,  as  well  as  all  the  utmost 
power  of  man  from  the  earliest  time.  It  is  trans- 
mutation, or  the  conversion  of  one  element  into 
another. 

Radioactivity  is  the  one  process  going  on  in 
matter  we  cannot  influence  or  stop,  while  trans- 
mutation is  the  one  process  in  matter  we  have 
so  far  signally  failed  to  effect.  The  juxtaposition 
of  radioactivity  and  transmutation  is  not  a  fanciful 
one,  because  it  will  appear,  as  we  proceed,  that 
the  two  processes  are  most  intimately  connected. 

Radioactivity  being  a  property  of  the  element 
uranium,  it  was  natural  to  ask  whether  uranium 
alone  of  all  the  eighty  elements  known  possessed 
it.  This  was  the  starting-point  of  Mme.  Curie's 
illustrious  researches  in  the  subject.  She  found 
only  one  other  element  among  those  known  which 
possessed  the  property  —  the  element  thorium, 
which,  at  one  time  rare  and  little  known,  has  come 
into  industrial  prominence  of  recent  years  in  the 
manufacture  of  the  Welsbach  incandescent  gas 
mantle,1  of  which  it  forms  the  main  constituent. 
To  the  electrical  test — the  power  of  discharging 

1  The  cause  of  the  action  of  the  gas-mantle  in  generating  light  is 
quite  unconnected  with  the  property  of  radioactivity. 


THORIUM  19 

a  gold-leaf  electroscope — thorium  preparations  are 
of  about  the  same  degree  of  radioactivity  as 
uranium  ;  but  to  the  photographic  plate  thorium 
is  far  less  active  than  uranium,  owing  to  the  fact 
that  the  type  of  rays  which  affect  the  photographic 
plate  most  strongly  are  not  those  with  most  effect 
on  the  electroscope.  The  radioactivity  of  thorium 
is  a  fact  which  can  be  beautifully  demonstrated  by 
any  one  acquainted  with  the  process  of  photo- 
graphy. An  incandescent  mantle,  after  burning  off 
the  fibre,  is  cut  open  and  pressed  as  flat  as 
possible  on  a  card.  A  photographic  plate,  which 
has  first  been  wrapped  in  a  light-tight  envelope, 
is  laid  upon  the  flat  mantle,  and  the  whole  is 
left  undisturbed  for  a  fortnight  or  longer.  On 
developing  the  plate  it  will  be  found  that  an  image 
of  the  mantle  has  been  formed  on  the  plate  in  the 
dark  by  the  rays  from  the  thorium  contained  in 
the  mantle.  Any  one  can  do  this  simple  experi- 
ment for  himself. 

Fig.  2  (facing  p.  10)  shows  the  result  I  obtained 
with  a  very  thin  piece  of  aluminium  foil  between 
the  film  and  the  mantle.  The  foil,  while  quite 
opaque,  allows  the  a-  as  well  as  the  /3-rays  to  go 
through.  Paper  would  stop  the  a-rays  entirely. 

The  radioactivity  of  thorium,  though  producing 
the  same  general  effects  as  that  of  uranium,  differs 
from  it  entirely  in  detail.  Indeed,  by  a  few  simple 


20  PITCHBLENDE 

tests  on  the  radioactivity,  any  one  of  the  radio- 
active elements  can  be  recognised  and  distinguished 
far  more  quickly  and  certainly  than  by  any  of  the 
other  chemical  or  spectroscopic  tests,  even  when 
present  in  very  minute  quantities.  In  the  his- 
torical development  of  the  views  now  held  in 
radioactivity  thorium  played  a  leading  part.  But, 
as  it  is  quite  foreign  to  my  intention  to  give  any- 
thing approaching  a  detailed  systematic  account 
of  the  subject,  and  as  radium  lends  itself  more 
readily  to  experimental  demonstrations,  I  shall 
confine  myself  primarily  to  the  properties  of  the 
latter  substance. 

Although  uranium  and  thorium  were  the  only 
two  known  elements  possessing  radioactivity,  Mme. 
Curie  found  that  the  natural  minerals  containing 
uranium  are  more  radioactive  than  can  be  accounted 
for  by  the  uranium  present.  Certain  minerals, 
called  pitchblende,  particularly  the  variety  from 
the  celebrated  Joachimsthal  mine  in  Austria,  con- 
tain often  more  than  50  per  cent  of  uranium  in 
the  form  of  uranium  oxide.  The  radioactivity  of 
pitchblende  to  the  photographic  plate  is  beautifully 
shown  by  two  photographs  of  Sir  W.  Crookes 
(Fig.  3).  The  lower  figure  shows  the  polished 
face  of  a  piece  of  pitchblende  photographed  in 
the  ordinary  way  by  daylight.  The  upper  figure 


FIG.  3.      Sir  William  Crookes'  Pictures  of  Pitchblende. 

The  lower  figure  is  a  daylight  photograph. 

The  upper  was  imprinted  in  the  dark  by  the  rays  from  the  substance. 


To  face  p.  20. 


PITCHBLENDE  21 

was  taken  by  placing  the  polished  face  of  the 
mineral  on  a  photographic  film  wrapped  in  light- 
tight  paper.  The  lighter  portions  of  the  figure 
indicate  where  the  plate  has  been  acted  on  by  the 
rays  from  the  radioactive  matter  in  the  pitchblende. 
Some  pitchblendes  are  from  three  to  four  times 
as  radioactive  as  pure  uranium  oxide.  This  could 
only  be  the  case,  Mme.  Curie  correctly  argued,  if 
there  existed  in  the  minerals  one  or  more  unknown 
elements  more  powerfully  radioactive  than  uranium. 
By  the  ordinary  process  of  chemical  analysis  it  is 
easy  to  separate  out  the  various  constituent  ele- 
ments in  pitchblende.  There  are  a  great  number 
of  elements  in  pitchblende,  though  most  of  them 
are  present  in  very  small  amount.  A  fact  that  will 
be  found  significant  later  is  that  lead  is  always 
present  in  important  quantity.  Mme.  Curie  found 
that  of  the  elements  so  separated  two  in  particular, 
the  bismuth  and  the  barium,  were  strongly  radio- 
active. Now  ordinary  bismuth  and  barium  are  not 
at  all  radioactive,  and  the  radioactivity  of  these 
elements,  when  separated  from  pitchblende,  is  really 
due  to  the  presence  of  two  new  elements  in  minute 
amount  mixed  with  them.  The  one  associated 
with  bismuth  was  discovered  first  by  Mme.  Curie 
and  named  Polonium,  after  her  native  country.  Its 
consideration  is  more  profitably  delayed  till  later. 
The  other,  which  was  discovered  very  soon  after- 


22     QUANTITY  OF  RADIUM  IN  PITCHBLENDE 

wards,    is   associated     with     the     barium    and    is 
Radium. 

The  exact  quantity  of  radium  in  pitchblende  and 
other  uranium  minerals  is  a  fact  of  considerable 
importance.  There  is  one  part  of  the  element 
radium  for  every  three  million  two  hundred  thousand 
parts  of  the  element  uranium  in  pitchblende.  The 
pitchblende  may  be  of  any  degree  of  richness,  from 
only  a  few  per  cent,  to  over  50  per  cent,  of  uranium. 
But  of  even  the  richest  pitchblendes  between  100 
and  200  tons  would  be  needed  to  produce  an  ounce 
of  pure  radium.  The  compound  usually  sold, 
radium  bromide,  contains,  if  pure,  58*5  per  cent. 
of  radium.  But  what  it  lacks  in  quantity  radium 
makes  up  for  in  quality — that  is  to  say,  in  radio- 
activity. It  is  like  the  myriad  of  roses  we  are  told 
go  to  make  a  single  drop  of  the  real  attar,  which  is 
almost  priceless.  The  radium  that  is  extracted  is  a 
million  times  more  radioactive  than  the  mineral,  and 
several  million  times  more  than  pure  uranium  itself. 
Conversely,  just  as  you  can  buy  quite  a  large  bottle 
of  rose-water  for  a  small  sum,  so  quantity  is  not 
the  only  consideration  to  be  taken  into  account  in 
the  buying  of  radium  preparations.  A  very  small 
quantity  of  radium  is  sufficient  to  confer  on  a  large 
quantity  of  an  inactive  salt  many  of  its  own  peculiar 
properties.  Particularly  is  this  the  case  with  the 
property  of  glowing  visibly  in  the  dark.  Weak 
radium  preparations,  which  contain  usually  barium, 


QUANTITY  OF  RADIUM  RECOGNISABLE      23 

shine  by  themselves  in  the  dark  more  strongly 
even  than  the  pure  radium  salts,  owing  to  a  phos- 
phorescent action  of  the  barium  salts,  although 
they  may  hardly  contain  enough  radium  to  affect 
an  X-ray  screen  through  a  piece  of  metal.  If  you 
mix  a  very  minute  quantity  of  radium  with  a 
quantity  of  a  very  highly  phosphorescent  body, 
like  sulphide  of  zinc,  it  will  shine  in  the  dark  so 
brilliantly  that  an  inexperienced  person  might  well 
be  deceived  into  believing  that  it  must  contain  a 
large  quantity  of  radium.  So  great  has  become 
the  need  that  radium  preparations  should  be  of 
definitely  ascertainable  quality  that  in  1910  an 
International  Radium  Standards  Committee  was 
formed,  with  the  result  that  there  is  now  pre- 
served in  Paris  an  International  Radium  Standard 
prepared  by  Mme.  Curie,  and  consisting  of  a  tube 
containing  twenty-two  milligrams  of  the  most  care- 
fully purified  radium  chloride.  This  standard  is  to 
form  the  basis  for  the  preparation  of  secondary 
standards  to  be  supplied  to  the  official  testing 
institutions  of  the  various  countries,  and  henceforth 
there  should  be  as  much  definiteness  about  the 
milligram  of  radium  as  there  is  about  a  pound  of  tea. 

It  is  an  interesting  digression  to  consider  here 
the  smallest  absolute  quantity  of  radium  which  can 
be  detected  and  identified  with  certainty  in  the 
laboratory.  One  fifty-millionth  of  a  milligram,  or 


24  EXPERIMENTS    WITH  RADIUM 

one  three- thousand-millionth  of  a  grain  of  radium 
is  quite  easy  to  recognise,  whilst  with  special  care 
one-tenth  of  this  amount  could  probably  be  detected. 
This  is  far  less  than  could  be  detected  in  the  case 
of  any  non-radioactive  element  by  any  method 
known,  not  excluding  even  the  spectroscope.  If 
the  half  of  a  grain  of  pure  radium  bromide,  which  is 
in  this  room  to-night,  were  divided  equally  among 
every  human  being  at  present  alive  in  the  world, 
and  one  such  portion  were  returned  to  us,  it  would 
prove  sufficient  for  detection  and  identification  by 
means  of  a  gold-leaf  electroscope  with  the  greatest 
ease.  With  half  a  grain  of  a  pure  radium  com- 
pound the  main  effects  of  radioactivity,  which  in  the 
case  of  uranium  or  thorium  would  either  be  too 
feeble  to  show  or  would  require  the  use  of  incon- 
veniently delicate  instruments,  can  be  shown  in  a 
striking  and  convincing  manner  to  you  all  in  the 
simplest  possible  way. 

Of  the  small  amount  of  radium  bromide,  which 
by  a  labour  of  love  certain  chemists  have  succeeded 
in  extracting  from  pitchblende,  I  am  fortunate  to 
possess  about  a  grain,  or  sixty-five  milligrams. 
Half  of  this  quantity,  which  I  shall  use  for  most  of 
my  lecture  experiments,  is  contained  in  a  small 
ebonite  capsule.  The  other  half  is  dissolved  in 
water  and  not  brought  into  this  lecture-room,  but 
kept  in  the  laboratory  half  a  mile  away.  With  the 


To  face  p.  25. 


EXPERIMENTS    WITH  RADIUM  25 

room  dark  the  radium  in  the  capsule  is  hardly 
visible  to  you,  because  the  rays  do  not  of  them- 
selves affect  the  unaided  eye,  but  if  I  bring  some 
crystals  of  the  fluorescent  substance,  barium  platino- 
cyanide,  near  to  it,  you  will  see  that  the  crystals 
shine  out  at  once  with  a  beautiful  green  light.  An 
ordinary  X-ray  fluorescent  screen,  which  is  simply 
a  piece  of  card  painted  over  with  the  same 
fluorescent  substance  in  the  form  of  powder,  is 
very  convenient  for  these  experiments.  When  thin 
pieces  of  metal  foil  are  placed  between  the  radium 
and  the  crystals  you  see  their  brightness  is  only 
slightly  reduced,  while  several  shillings  can  be 
interposed  one  above  the  other  without  altogether 
stopping  the  rays  from  the  radium.  Those  in  the 
front  will  see  the  crystals  still  shining  faintly, 
although  the  rays  from  the  radium  have  first  to 
traverse  more  than  half  an  inch  of  solid  silver 
before  reaching  the  crystals.  The  electrical  effect 
oi  radioactivity  can  be  shown  in  a  very  rough  and 
simple  way  with  this,  comparatively  speaking,  large 
quantity  of  radium.  A  silk  tassel  is  stroked  with  a 
rubber  tobacco-pouch  and  so  electrified.  All  the 
threads  then  repel  one  another  and  stand  out  as 
you  see  (Fig.  4).  The  moment  the  radium  is 
brought  near  the  threads  collapse  at  once  (Fig.  5). 
Lastly,  the  photographic  action  of  the  rays  is  seen 
in  the  photograph  (Fig.  6,  facing  p.  44)  which  was 
obtained  by  slowly  writing,  with  a  small  tube  con- 


26  VALUE  OF  RADIUM 

taining  a  small  fraction  of  a  grain  of  radium  bromide 
as  if  it  were  a  pencil,  over  a  photographic  plate 
wrapped  in  black  paper,  and  then  developing  the 
plate  without  exposure  to  light. 

By  the  aid  of  delicate  thermometers  it  could  also 
be  shown  that  this  small  quantity  of  radium  is  always 
a  few  degrees  hotter  than  the  surrounding  air. 

The  one  fact  about  radium,  which  every  one  is 
aware  of,  is  its  tremendous  cost.  When  you  con- 
sider that  even  of  the  best  ore  several  hundred- 
weights must  be  worked  up  to  obtain  the  small 
quantity  here  exhibited,  you  can  understand  that 
the  price  is  necessarily  very  high.  The  price  rose 
rapidly  from  about  8s.  the  milligram  for  radium 
bromide  in  1903  to  about  £\$  the  milligram  in 
1912,  and  even  at  the  latter  price  very  inferior 
preparations  have  found  a  ready  sale.  We 
shall  see,  as  we  proceed,  that  from  its  very  nature 
any  strongly  radioactive  body  like  radium  must 
always  be  excessively  rare.  Indeed,  in  the  degree 
of  radioactivity  we  have  a  scientific  standard  of 
rarity,  and  therefore  of  "  value."  There  are  un- 
fortunately some  fields  of  scientific  investigation, 
of  which  radioactivity  is  one,  which  cannot  be 
thoroughly  explored  without  continuous  and  con- 
siderable expenditure.  The  old  boast  of  science, 
that  some  of  her  grandest  discoveries  were  made 
with  very  simple  apparatus,  largely  built  up  of  wire 


A  SCIENTIFIC  MONOPOLY  27 

and  sealing-wax,  costing  little  or  nothing,  does  not 
apply  to  any  of  the  discoveries  with  which  we  are 
now  concerned.  The  investigations  of  Mme.  Curie 
naturally  have  cost  many  thousands  of  pounds,  pro- 
vided in  part  by  the  Austrian  Government  and  the 
Rothschilds.  This  radium  we  are  using  to-night  we 
owe  to  the  work  of  a  German  chemist,  Dr.  Giesel, 
who  undertook  its  extraction  on  a  large  scale  in  the 
early  days  when  the  raw  material  was  to  be  obtained 
in  the  market,  and  who  very  unselfishly  distributed 
much  of  the  radium  he  prepared  among  workers  in 
all  parts  of  the  world.  To-day  the  Austrian  Govern- 
ment, who  hold  the  monopoly  of  the  Joachimsthal 
mine,  do  not  allow  the  ore  to  be  exported,  and  there 
is  a  great  dearth  of  the  raw  material.  Even  those 
who  have  borne  the  heat  and  burden  of  the  day, 
who  have  given  the  waste  raw  material  its  value, 
and  who  have  discovered  the  very  methods  of 
extraction,  are  unable  any  longer  to  obtain  the 
raw  material  for  their  researches.  A  monopoly  in 
scientific  research  is  an  unfortunate  innovation,  but 
such,  alas  !  so  limited  is  the  supply  of  radium,  exists 
to-day.  The  Royal  Society  recently  obtained,  as 
a  privilege,  a  thousand  pounds'  worth  of  the  raw 
material,  but  it  produced  less  than  fifty  milligrams 
of  radium. 

To-night   it   is    not    my    intention    to   take   you 
through  the  various  phases  of  the  new  properties 


28  DOCTRINE  OF  ENERG  Y 

of  radium.  We  have  to  face  squarely  the  great 
general  question  which  its  simple  existence  has 
demanded  of  physical  science.  Last  century  will 
remain  for  ever  memorable  on  account  of  the  de- 
velopment and  establishment  of  the  great  doctrine 
of  energy.  Those  were  great  days  for  physical 
science  in  Scotland,  for  that  doctrine,  which  lies  at 
the  root  of  all  modern  industry  and  enterprise,  took 
its  rise  largely  in  Scotland,  and  was  developed  by 
Tait,  of  Edinburgh,  and  Lord  Kelvin,  of  Glasgow. 
For  a  full  account  of  these  stirring  developments 
you  should  read  Tait's  Recent  Advances  in  Physical 
Science,  which,  in  spite  of  the  fact  that  it  is  now  over 
thirty  years  old,  still  continues  fresh  and  inspiring. 
The  first  law,  that  of  the  conservation  of  energy,  states 
that  energy  is  a  real  entity,  and  has  a  real  existence 
no  less  than  matter,  and  no  more  than  matter  can 
energy  be  created  or  destroyed,  although  the  forms 
it  may  assume  are  legion.  The  second  law,  that  of 
the  availability  of  energy,  is  sufficiently  accurately 
stated  for  present  purposes  by  saying  that  the  same 
energy  is  available  for  useful  work  but  once.  To 
obtain  useful  work  from  any  source  of  stored- up  or 
potential  energy,  it  is  necessary  to  transform  it  into 
new  forms  which  are  kinetic,  and  by  which  some- 
thing is  made  to  move.  As  motion  is  invariably 
attended  by  friction  or  similar  processes,  ultimately 
the  energy  passes  into  heat.  It  is  said  to  be  de- 


DOCTRINE   OF  ENERGY  29 

graded  into  low-grade  or  waste  energy,  for  although 
all  forms  of  energy  tend,  after  assuming  the  kinetic 
form,  to  turn  into  heat,  the  transformation  of  the 
waste  heat  so  produced  back  into  useful  forms  can- 
not be  practically  effected.  The  conversion  is  not 
altogether  impossible,  but  requires  for  its  accom- 
plishment the  degradation  of  more  fresh  energy  than 
is  gained,  and  so  is  practically  out  of  the  question. 

The  practical  aspect  of  the  question  may  be 
summed  up  by  saying  that  if  you  want  useful  energy 
you  must  pay  for  it  like  any  other  commodity,  and 
the  value  of  the  energy,  though  not  the  energy 
itself,  is  destroyed  by  use.  The  up-to-date  street 
car  driven  by  the  electric  motor,  which  has  displaced 
the  old  horse-tram,  although  it  has  not  the  same 
obvious  incentive  to  locomotion  as  its  predecessor, 
nevertheless  does  not  go  by  itself.  It  requires 
energy  or  power,  which  is  bought  and  sold  and 
has  a  value  as  strictly  as  the  oats  and  hay  which 
energised  the  now  emancipated  horse.  The  driving 
power  of  the  machinery  of  the  modern  world  is  often 
mysterious,  but  the  laws  of  energy  state  that  nothing 
goes  by  itself,  and  our  experience,  in  spite  of  all  the 
perpetual  motion  machines  which  inventors  have 
claimed  to  have  constructed,  bore  this  doctrine  out, 
until  we  came  face  to  face  with  radium.  Nothing 
goes  by  itself  in  Nature,  except  apparently  radium 
and  the  radioactive  substances.  That  is  why,  in 


30  DOCTRINE  OF  ENERG  Y 

radioactivity,  science  has  broken  fundamentally  new 
ground. 

I  cannot  too  plainly  insist  that  available  energy, 
though  immaterial  and  intangible,  has  a  definite  and 
real  physical  existence.  Were  it  not  so,  coal  would 
not  be  the  very  expensive  commodity  it  unfortu- 
nately is  rapidly  becoming.  No  one  burns  coal  for 
the  sake  of  polluting  the  atmosphere,  but  simply 
and  solely  because  it  gives  out  during  combustion 
a  certain  amount  of  energy  as  light  or  heat.  Last 
century  civilisation  may  be  said  to  have  attained  its 
majority  and  to  have  entered  upon  the  control  of 
an  inheritance  of  energy  stored  up  by  the  sun  in 
fuel  during  the  long  ages  of  the  past,  and  now  it  is 
dissipating  that  inheritance  as  quickly  as  it  can. 
With  the  lightheartedness  and  irresponsibility  of 
youth,  it  is  taking  no  thought  of  the  future, 
but  confidently  assumes  that  the  supply  of  natural 
energy,  upon  which  at  every  turn  it  is  now  entirely 
dependent,  will  continue  indefinitely.  Well !  if  it 
does  not  do  so,  new  stores  of  energy  cannot  be 
created  to  order,  and  there  will  be  an  end  to  the 
age  of  energy  in  which  we  are  living,  and  to  civili- 
sation as  we  have  come  to  understand  it. 

Energy  is  susceptible  of  exact  measurement  and, 
though  it  exists  in  many  varieties,  all  forms  of 
energy  can  be  most  readily  and  completely  con- 


ENERGY  EVOLVED  BY  RADIUM  31 

verted  into  heat  and  measured  as  such.  The 
energy  given  out  by  radium,  although  it  is  in 
nature  new,  is  no  exception  to  this  rule.  Practically 
the  whole  of  the  energy  is  transformed  into  heat 
when  the  radium  is  kept  in  a  leaden  vessel,  so  that 
the  rays  are  absorbed  in  the  surrounding  metal. 
The  actual  amount  of  heat  given  out,  for  instance, 
by  this  small  quantity  on  the  table  is,  of  course,  very 
small,  but  in  comparison  with  the  quantity  of  sub- 
stance producing  it,  it  is  very  great  indeed.  Exact 
experiments  have  proved  that  i  gram  ( =  15*4  grains) 
of  radium  gives  out  133  calories  per  hour.  The 
amount  of  heat  evolved  by  any  quantity  of  radium 
in  three-quarters  of  an  hour  is  as  much  as  is 
required  to  raise  a  quantity  of  water  equal  in  weight 
to  the  radium  from  the  freezing-point  to  the  boiling- 
point.  Radium  bromide,  if  it  is  pure,  consists 
roughly  of  three-fifths  by  weight  of  the  element 
radium  and  two-fifths  of  the  element  bromine. 
Half  a  grain  of  radium  bromide  thus  evolves  two 
and  a  half  calories  every  hour.  This  specimen  of 
half  a  grain  of  radium  bromide  has  been  in  my 
possession  for  nine  years,  and  the  outpouring  of 
energy  has  been  going  on  ceaselessly  day  and  night 
at  a  steady  rate.  A  simple  calculation  shows  that 
in  this  time  about  200,000  calories  have  been  evolved. 
To  obtain  an  idea  of  what  this  means  consider  the 
amount  of  energy  given  out  in  the  burning  of  coal. 
A  weight  of  coal  equal  to  the  weight  of  this  radium 


32  ENERGY  EVOLVED  BY  RADIUM 

bromide  would  give  out  during  complete  combustion 
only  about  250  calories,  so  that  this  radium  has 
evolved  in  nine  years  800  times  the  energy  obtain- 
able from  the  same  weight  of  coal.  I  have  chosen 
coal  for  the  comparison  because  the  combustion  of 
carbon  furnishes  the  modern  world  with  its  main 
supply  of  energy.  During  the  last  nine  years  this 
radium  has  given  hundreds  of  times  as  much  energy 
as  could  be  obtained  from  an  equal  weight  of  any 
other  kind  of  substance  in  any  way  known.  Coal  is 
no  longer  coal  when  it  is  burnt  and  consumed.  Gun- 
powder and  dynamite,  once  they  have  exploded  and 
evolved  their  stored-up  energy,  disappear  as  such, 
and  there  remain  incombustible  and  non-explosive 
solids  and  gases,  out  of  which  no  more  energy  can 
be  drawn.  But  this  radium  is  as  active  as  ever.  So 
far,  careful  measurements  have  failed  to  detect  the 
least  diminution  in  the  radioactivity  of  radium  with 
time.  Rather  it  increases  steadily,  rapidly  in  the 
first  month  and  slowly  for  the  first  few  years  after 
preparation,  for  certain  profound  reasons  we  shall 
have  to  go  into  subsequently. 

In  the  face  of  a  new  fact  of  this  character  it  is 
obvious  that  this  doctrine  of  energy,  which  we 
thought  so  well  founded,  requires  further  considera- 
tion. Based  as  it  has  always  been  on  the  results  of 
our  experience  and  the  practical  impossibility  of 


COSMIC AL  ENERGY  33 

achieving  perpetual  motion  of  any  kind,  it  is  con- 
fronted with  a  natural  example,  going  on  apparently 
for  an  unlimited  space  of  time  under  our  very  eyes, 
which  not  only  does  not  come  to  a  stop,  but  which 
cannot  be  stopped  by  any  means  whatever.  Now, 
although  the  doctrine  of  energy  accords  well 
enough  with  our  terrestrial  experiences,  the  student 
of  the  physical  sciences  has  only  to  turn  his 
thoughts  from  the  laboratory  to  the  heavens  to 
see  there,  in  the  larger  laboratory  of  Nature,  an 
example  of  practical  perpetual  motion  on  the 
grandest  and  most  majestic  scale.  What,  for  ex- 
ample, is  the  source  of  the  apparently  inexhaustible 
supply  of  energy  from  the  sun,  upon  the  receipt  of 
a  minute  and  insignificant  fraction  of  which  life  on 
this  planet  absolutely  depends  for  its  continued 
existence  from  year  to  year?  This  is  a  question 
which  has  been  frequently  asked  and  only  im- 
perfectly answered  by  physical  science.  It  has 
been  the  custom  vaguely  to  connect  the  appar- 
ently endless  and  inexhaustible  outpourings  of 
energy  going  on  everywhere  in  the  universe  with 
its  vast  scale  and  dimensions.  In  the  background 
there  has  always  been  the  tacit  assumption  that 
the  supply  of  fresh  energy  is  only  apparently 
inexhaustible,  and  that  in  some  remote  future 
a  time  will  at  length  arrive  when  the  supplies 
of  fresh  energy  are  exhausted  and  all  things 


34         QUOTATION  FROM  PROFESSOR   TAIT 

will  come  to  a  stop  and  remain  at  rest  for 
ever.  We  have  applied  the  teachings  of  the 
laboratory,  our  knowledge  of  the  laws  of  energy 
and  its  conservation,  and  the  impossibility  of 
perpetual  motion,  without  modification  to  the 
cosmos,  only  making  allowance  for  its  enormous 
scale. 

Astronomers,  if  one  may  judge  from  recent 
utterances,  still  continue  to  regard  cosmical  evolu- 
tion as  proceeding  on  these  old  conventional  lines. 
That  is,  they  seem  to  regard  it  still  as  only  a 
question  of  time  before  the  sun  and  planets  cool 
down  to  a  dead  uniform  temperature.  In  former 
days  this  point  of  view  was  the  only  possible  one. 
A  hot  body  radiating  heat  and  light  into  space, 
even  when  all  possible  sources  of  energy,  such  as 
the  accretion  of  meteorites,  shrinkage,  etc.,  have 
been  allowed  for,  must  ultimately  radiate  away  its 
energy.  The  same  is  still  true  but  with  a  differ- 
ence. Thus  Professor  Tait,  in  his  Recent  Advances 
in  Physical  Science  (1876),  says  (p.  169) :  "  If  we 
were  to  trace  the  state  of  affairs  back,  instead  of  to 
ten  millions,  to  a  hundred  millions  of  years,  we 
should  find  that  (if  the  earth  then  existed  at  all) 
if  that  collocation  of  matter  which  we  call  the 
earth  was  then  actually  formed,  and  if  the  physical 
laws  which  at  present  hold  have  been  in  operation 
during  that  hundred  million  years,  then  the  surface 


QUOTATION  FROM  PROFESSOR   TAIT        35 

of  the  earth  would  undoubtedly  have  been  liquid 
and  at  a  high  white  heat,  so  that  it  would  have 
been  utterly  incompatible  with  the  existence  of  life 
of  any  kind  such  as  we  can  conceive  from  what  we 
are  acquainted  with.  Thus  we  can  say  at  once  to 
geologists,  that  granting  this  premiss — that  physical 
laws  have  remained  as  they  now  are,  and  that  we 
know  of  all  the  physical  laws  which  have  been 
operating  during  that  time — we  cannot  give  more 
scope  for  their  speculations  than  about  ten  or  (say  at 
most)  fifteen  millions  of  years. 

"  But  I  dare  say  many  of  you  are  acquainted  with 
the  speculations  of  Lyell  and  others,  especially  of 
Darwin,  who  tell  us  that  even  for  a  comparatively 
brief  portion  of  recent  geological  history  three 
hundred  millions  of  years  will  not  suffice. 

"  We  say,  so  much  the  worse  for  geology  as  at 
present  understood  by  its  chief  authorities,  for,  as 
you  will  presently  see,  physical  considerations  from 
various  independent  points  of  view  render  it  utterly 
impossible  that  more  than  ten  or  fifteen  millions  of 
years  can  be  granted.' 

Again  (p.  154):  "Take  (in  mass  equal  to  the 
sun's  mass)  the  most  energetic  chemicals  known  to 
us,  and  in  proper  proportion  for  giving  the  greatest 
amount  of  heat  by  actual  chemical  combination, 
and,  so  far  as  we  yet  know  their  properties,  we 
cannot  see  the  means  of  supplying  the  sun's  present 


36        NEW  STORES   OF  NATURAL   ENERGY 

waste  for  even  5000  years.  .  .  .  This  question 
is  totally  unanswerable,  unless  there  be  chemical 
agencies  at  work  in  the  sun  of  a  far  more  power- 
ful order  than  anything  that  we  meet  with  on  the 
earth's  surface." 

I  do  not  quote  these  utterances  with  any  wish  to 
revive  the  old  controversy  between  geologists  and 
physicists,  long  since  tacitly  abandoned  by  both 
sides  mutually  as  barren  and  unprofitable,  but 
because  of  their  present  extraordinary  aptness. 
To-day,  science  has  come  to  know,  by  means  of 
radioactivity,  of  agencies  at  work  on  the  earth's 
surface  of  a  far  more  powerful  order  than  anything 
that  was  known  in  the  time  of  Professor  Tait. 
The  discovery  of  radioactivity  and  the  revelation 
it  has  given  of  unsuspected  stores  of  energy  in 
Nature  available  for  cosmical  purposes,  of  necessity 
put  the  whole  question  of  the  evolution,  the  past 
history  and  the  future  destiny  of  the  universe  in  a 
new  light.  This  is  one  of  the  conclusions  of 
clearly  general  interest  which  follow  from  the 
recent  discoveries. 

There  is  nothing  of  the  vast  scale  and  dimensions 
of  the  universe  about  this  tiny  scrap  of  radium. 
Yet  it  is  giving  out  energy  at  a  rate,  relative  to  its 
mass,  which  no  sun  or  star  is  doing.  Suppose,  for 
example,  our  sun,  instead  of  being  composed  of  the 


THE  INDIFFERENCE   OF  RADIUM  37 

materials  it  is,  which  we  know  by  the  spectroscope 
are  practically  the  same  as  those  of  the  earth,  were 
made  of  pure  radium.  Provided  only  that  every 
part  of  its  mass  gave  out  energy  at  the  rate  this 
radium  on  the  table  is  doing,  there  would  then  be 
no  difficulty  in  accounting  for  its  outpourings  of 
energy.  Rather,  the  light  and  heat  that  would  be 
given  out  from  such  a  sun  would  be  of  the  order  of 
a  million  times  greater  than  they  actually  are.  On 
another  count  also  one's  thoughts  almost  uncon- 
sciously revert  from  radium  to  the  transcendental 
phenomena  of  the  larger  universe,  for  in  no  other 
phenomena  are  we  so  reduced  to  the  position  of 
onlookers,  powerless  alike  to  influence  or  control. 
All  the  powerful  resources  of  the  modern  labora- 
tory— extremes  of  heat  and  cold,  and  of  pressure, 
violent  chemical  reagents,  the  action  of  powerful 
explosives  and  the  most  intense  electrical  dis- 
charges— do  not  affect  the  radioactivity  of  radium 
or  the  rate  at  which  it  works  in  the  slightest 
degree.  It  draws  its  supplies  of  energy  from  an 
hitherto  unknown  source  and  obeys  as  yet  un- 
discovered laws.  There  is  something  sublime 
about  its  aloofness  from  and  its  indifference  to 
its  external  environment.  It  seems  to  claim 
lineage  with  the  worlds  beyond  us,  fed  with  the 
same  inexhaustible  fires,  urged  by  the  same  un- 
controllable mechanicism  which  keeps  the  great 


38  "PHYSICAL  IMPOSSIBILITY" 

suns  alight  in  the  heavens  over  endless  periods 
of  time.  This  tiny  speck  of  matter  we  can  hold 
in  our  hands  exhibits  in  perfect  miniature  many 
ancient  mysteries,  forgotten  almost  in  their 
familiarity,  or  mistakenly  and  too  easily  dismissed 
as  belonging  and  appropriate  to  the  infinitely  great 
dimensions  of  the  universe.  The  "  physical  im- 
possibility "  of  one  era  becomes  the  commonplace 
of  the  next,  and  in  the  controversy  between  the 
geologists  and  the  physicists  we  have  a  good 
illustration  that  no  theory  can  claim  a  universal 
application.  It  is  of  necessity  partial,  and  bounded 
on  all  sides  by  the  unknown  and  unexplored.  It 
is  rarely  proved  false,  so  surely  and  truly  are  the 
foundations  of  modern  science  laid,  but  it  is 
liable  at  any  moment  to  be  restricted  in  its  appli- 
cation to  the  particular  cases  for  which  it  was 
formulated  and  found  not  to  apply  in  new  spheres 
at  the  time  of  its  inception  unsuspected.  As  we 
shall  see,  the  law  of  the  conservation  of  energy 
is  not  necessarily  controverted  by  any  of  the 
new  facts  with  reference  to  radium,  but  prior  to 
these  discoveries  our  knowledge  of  the  avail- 
able sources  of  energy  in  Nature  has  been  partial 
and  superficial  to  a  degree. 


CHAPTER    III. 

The  radiations  of  the  radio-elements — a-,  /3-,  and  7-rays — Test  of 
penetrating  power — Experiments  with  the  penetrating  /3-  and 
-y-rays — The  non-penetrating  a-rays — Experiment  to  show  the 
absorption  of  a-rays  by  air — Physical  nature  of  radiations — Neces- 
sity for  an  ether — Corpuscular  and  wave  radiations — a-  and  /3-rays 
due  to  the  ex'pulsion  of  particles — Can  a  single  a-particle  be  de- 
tected?— The  spinthariscope — Counting  the  a-particles  expelled 
by  radium. 

IN  the  previous  lectures  we  have  considered  the 
bare  fact  that  radium  and  the  radioactive  sub- 
stances are  continually  evolving  from  themselves 
a  perennial  supply  of  energy,  and  the  fundamentally 
new  ground  which  this  discovery  opens  up  in 
physical  science.  To-night  our  inquiries  will  be 
directed  to  one  special  portion  of  the  subject,  namely, 
the  nature  of  the  rays  emitted  by  the  radioactive 
elements,  by  means  of  which,  or  rather  of  the 
effects  of  which,  the  property  was  first  discovered. 
These  rays  themselves,  apart  from  their  effects,  we 
have  hitherto  scarcely  considered,  but  they  play  an 
essential  part  in  the  theoretical  scheme  by  which 
the  activity  of  the  radio-elements  is  now  inter- 
preted. 

The  tracing  back  of  the  main  effects  of  radio- 

39 


40        RAYS  OF  RADIOACTIVE   SUJ3STANCES 

activity,  photographic,  fluorescent,  electrical,  and 
thermal,  to  definite  radiations  emitted  by  the  radio- 
elements  came  very  early  in  the  subject,  but  it  must 
not  be  forgotten  that  such  tracing  back  is  of  the  es- 
sence of  the  discovery.  Too  frequently  it  is  wrongly 
assumed  without  such  evidence  that  any  substance 
capable  of  simulating  one  or  other  of  the  various 
effects  of  radioactivity  is  therefore  a  radioactive 
substance.  Naturally,  the  exact  study  of  the  new 
radiations  has  been  mainly  the  work  of  physicists. 
They  have  succeeded,  not  only  in  clearly  analysing 
into  distinct  classes  the  complex  radiations  involved 
and  distinguishing  the  part  played  by  each  alone, 
but  also  they  have  advanced  very  far  towards  a 
solution  of  the  real  nature  of  each  class  of  radiation 
emitted.  Much  of  this  latter  work,  however,  is 
based  upon  reasoning  of  too  specialised  and  in- 
tricate character  for  general  presentation,  and  as 
these  lectures  are  intended  primarily  for  the  general 
public,  and  not  for  trained  physicists,  I  propose  con- 
centrating attention  for  the  most  part  on  the  con- 
clusions which  are  universally  accepted  and  of  the 
greatest  general  interest.  Although  the  reasoning 
is  difficult,  the  chief  conclusions  are  very  simple  and 
easily  followed,  and  they  fit  in  with  the  general 
scheme  of  the  cause  and  nature  of  radioactivity  in 
a  way  which  makes  the  whole  subject  clearer  and 
more  easily  visualised. 


a-,   /?-,    AND   y-RAYS  41 

The  first  analysis  of  the  complex  radiations 
emitted  by  each  of  the  radio-elements — uranium, 
thorium,  and  radium — was  done  by  Rutherford,  and 
much  of  the  work  we  are  considering  is  his,  and 
has  called  forth  in  their  highest  degree  his  well- 
known  experimental  genius  and  energy.  He 
classed  the  rays  into  three  main  types,  the  a-,  8-, 
and  y-,  distinguished  from  one  another  by  enormous 
differences  in  their  power  of  penetrating  matter. 
I  may  say  at  once  that  the  a-rays  of  radium,  for 
instance,  are  readily  distinguishable  in  penetrating 
power  from  the  a-rays  of  uranium,  and  the  latter 
again  from  those  of  thorium.  Moreover,  the  a-rays 
of  radium  are  themselves  complex  and  consist  of 
no  less  than  four  separate  types  readily  distin- 
guished. The  same  is  true  of  the  ft-  and  y-rays 
of  radium,  which  are  themselves  complex  and 
recognisably  different  from  the  /3-  and  y-rays  of 
uranium  or  thorium.  But  the  differences  between 
the  a-rays  as  a  class,  for  example,  are  small  and 
unimportant  relatively  compared  to  the  enormous 
difference  between  any  a-ray  and  any  /3-ray  or 
y-ray.  The  most  penetrating  a-ray  known  is  not 
much  more  than  twice  as  penetrating  as  the  least 
penetrating  known,  whereas  the  /3-rays  as  a  class 
may  be  considered  to  be  approximately  a  hun- 
dred times  more  penetrating  than  the  a-,  and  the 
y-rays  a  hundred  times  more  penetrating  than 


42  PENETRATION  TEST 

the  /3-.  Again,  the  kind  of  matter  penetrated, 
although  it  has  a  certain  influence  which  may  be 
different  for  different  types  of  rays,  is  only  of 
secondary  importance.  For  these  rays,  like  the 
new  X-rays,  and  unlike  light,  are  absorbed  by 
matter  roughly  in  proportion  to  its  density,  and 
quite  independently  of  its  optical  qualities  of  trans- 
parency and  opacity.  The  first  result  of  these  re- 
searches was  to  bring  into  prominence  the  a-class 
of  rays,  which  at  first  sight  are  of  apparently  little 
importance,  and  to  diminish  relatively  the  impor- 
tance of  the  #-class  of  rays  which  had  been  operative 
in  the  photographic  effects  hitherto  mainly  studied. 
For  the  a-rays  are  completely  absorbed  by  very 
thin  screens — even  by  a  sheet  of  thin  paper,  or  by 
three  inches  of  ordinary  gaseous  air, — and  they  pro- 
duce but  little  action  on  the  photographic  plate  in 
comparison  with  the  /3-rays,  which  are  able  to  pass 
through  a  visiting  card  or  piece  of  tinfoil  with  ease. 
To  the  electrical  test — the  discharge,  for  example, 
of  an  electrified  silk  tassel  or  electroscope — the 
a-rays  are  immensely  more  effective  than  the  /3-  and 
y-rays  together,  and  from  this  fact  Rutherford  con- 
cluded, and  the  conclusion  has  been  wholly  borne 
out  by  subsequent  developments,  that  the  energy 
possessed  by  these  feebly  penetrating,  and  not  at 
first  sight  very  striking,  a-rays  is  always  immensely 
greater  than  that  of  the  other  two  types  taken 


EXPERIMENTS    WITH  ft-  AND  y-RAYS        43 

together.  In  fact,  the  ft-  and  y-rays  at  most  possess 
but  a  few  per  cent  of  the  total  energy  of  radiation, 
and  therefore  are  in  this  fundamental  respect  rela- 
tively of  less  consequence  than  the  previously 
neglected  a-class.  Although  less  suited  to  lecture 
experiments  than  the  other  more  penetrating  types, 
the  a-class  have  proved  far  the  most  instructive  and 
important  in  the  theory  of  radioactive  change. 

The  small  capsule  in  which  my  radium  is  con- 
tained is  closed  by  a  thin  sheet  of  mica,  which 
effectively  stops  all  the  a-rays,  so  that  in  working 
with  the  capsule  only  the  ft-  and  y-rays  are  opera- 
tive. The  platinocyanide  salts  fluoresce  most 
brilliantly  under  the  /3-rays.  On  interposing  suc- 
cessive thicknesses  of  thin  copper  or  aluminium  foil 
the  fluorescence  is  weakened,  very  rapidly  at  first, 
but  a  point  is  soon  reached  when  the  feeble  fluor- 
escence remaining  is  not  much  further  weakened 
by  additional  thicknesses  of  foil.  This  is  because 
the  /3-rays  have  all  been  absorbed,  and  there  re- 
main only  the  relatively  feeble  but  extraordinarily 
penetrating  y-rays.  These  y-rays  are  always  very 
feeble,  and  comparatively  unimportant,  but  their 
chief  interest  lies  in  the  fact  that  they  are  by  fai 
the  most  penetrating  type  of  radiation  at  present 
known.  If  the  capsule  is  completely  closed  in  a 
box  of  steel,  half  an  inch  thick,  and  a  platino- 


44  y-RAYS 

cyanide  crystal  laid  on  the  top,  those  in  front  can 
readily  see  that  the  crystal  still  fluoresces,  and 
stops  the  moment  it  is  taken  away  from  the  radium. 
Through  a  pile  of  twelve  shillings,  or  pennies,  the 
effect  can  still  be  observed,  while  by  means  of  a 
sensitive  gold-leaf  electroscope  it  can  readily  be 
shown  that  some  of  the  rays  still  penetrate  a  foot 
thickness  of  solid  iron,  or  six  inches  of  lead. 

The  rays  from  radium  are  not  well  adapted  for 
the  taking  of  radiographs  of  the  kind  produced  by 
X-rays.  The  /3-rays  are  hardly  sufficiently  pene- 
trating for  this  purpose,  so  that  the  flesh  as  well  as 
the  bones  of  the  hand,  for  example,  casts  a  heavy 
shadow.  The  y-rays,  on  the  other  hand,  are  far 
too  penetrating,  and  the  bones  hardly  cast  a 
shadow  at  all.  The  picture  (Fig.  7),  however,  is 
a  good  example  of  a  radium  radiograph  taken  by 
the  y-rays  of  radium.  A  small  box  of  compasses  with 
the  lid  shut  was  placed  on  a  table.  Over  it,  film 
down,  was  placed  an  X-ray  plate  wrapped  in  a  light- 
tight  envelope.  On  the  floor  beneath,  at  a  distance 
of  twenty-five  inches  from  the  plate,  was  placed 
one-tenth  of  a  grain  of  pure  radium  bromide  sealed 
up  in  a  tiny  glass  tube.  The  radium  was  placed 
between  the  poles  of  an  electro-magnet,  as  recom- 
mended by  Mme.  Curie,  to  deflect  away  the 
/3-rays  which  tend  to  blur  the  distinctness  of  the 
picture.  In  this  way  the  y-rays  of  radium  were 


FIG.  6.     Written  by  Radium  in  the  Dark. 
(From  a  Radiograph  by  R.  Hill  Crombie,  Esq.,  Journal  of  tJie  Rontgen  Society,  Dec.,  1906.) 


FIG.  7.     Closed  Box  of  Compasses  taken  with  the  7- Rays  of  Radium. 

To  face  p. 


y-RA  YS  45 

alone  used.  The  exposure  was  five  days.  It  will 
be  seen  that  the  shadow  cast  by  the  wooden  box  is 
scarcely  noticeable,  while  even  the  metal  compasses 
and  fastenings  of  the  box  by  no  means  entirely 
stop  the  rays.  The  metal  parts  appear  in  the 
negative  only  slightly  darker  than  the  unprotected 
portions  of  the  plate.  The  negative  was  reduced 
and  intensified  before  reproduction. 

At  first  the  y-rays  appeared  to  be  a  secondary 
radiation  produced  by  and  accompanying  the  j3-rays, 
much  as  X-rays  are  produced  by  and  accompany 
cathode-rays.  The  j3-  and  -y-rays  seemed  always 
to  go  together,  any  variation  of  the  /3-rays  being 
accompanied  by  a  similar  variation  of  the  -y-rays. 
This  is  now  known,  however,  not  to  be  invariably 
the  case,  and  the  opinion  is  gaining  ground  that  the 
/3-  and  «y-rays  are  not  necessarily  connected.  The 
real  nature  of  the  -y-rays  is  at  present  an  open  ques- 
tion as  rival  views  are  in  the  field,  and  as  the  rays 
are  not  of  primary  importance  at  the  present  time  we 
may,  with  these  experiments  and  remarks,  dismiss 
the  subject  and  pass  on  to  the  more  detailed  con- 
sideration of  the  two  more  important  types  of  rays. 

Before  proceeding  to  show  experiments  with  the 
a-rays,  it  is  necessary  to  touch  on  certain  considera- 
tions which  come  into  play  on  account  of  their 
very  great  absorption  in  passing  through  matter. 
In  the  first  place,  radioactivity  is  a  mass  or  volume 


46  a-RA  YS 

phenomenon.  That  is  to  say,  every  part,  not 
the  surface  only  but  the  inner  portions  also,  of  a 
radium  salt,  for  example,  is  giving  out  a-,  ft-  and 
y-rays.  All  these  rays  are  absorbed  by  the  sub- 
stance itself  very  considerably,  for  the  salts  of 
radium  are  dense  or  heavy.  But  this  absorption 
naturally  does  not  affect  the  more  penetrating  rays 
nearly  so  much  as  the  feebly  penetrating  a-rays. 
That  part  of  the  latter,  generated  inside  the  salt, 
does  not  escape  at  all.  Only  a  very  thin  surface 
film  contributes  to  the  a-radiation.  The  conse- 
quence is  that  whereas,  with  the  small  quantities  of 
radium  that  we  have  to  work  with,  the  strength  of 
the  penetrating  rays  is  more  or  less  proportional 
to  the  quantity  of  radium  employed,  with  the 
a-rays  this  is  no  longer  the  case.  The  weight  of  the 
substance  is  less  important  than  the  amount  of  sur- 
face exposed.  A  very  small  quantity,  say  a  milli- 
gram, of  radium  bromide,  spread  out  as  a  thin 
film  on  a  large  plate,  will  give  out  immensely  more 
a-rays  than  the  same  quantity  in  the  form  of  a  small 
crystal.  In  order  to  free  the  ft-  and  y-rays  from  the 
a-rays,  or  the  y-rays  from  the  /3-rays,  it  suffices  to 
interpose  screens  of  successively  increasing  thick- 
ness until  the  more  easily  stopped  type  is  completely 
absorbed.  But  it  is  not  possible  so  easily  to  eliminate 
by  physical  methods  the  ft-  and  y-rays  from  the 
a-rays  in  order  to  leave  the  latter  by  themselves. 
For  practical  purposes,  however,  this  result  can  be 


a-RA  YS  47 

achieved  very  simply.  If  we  take  a  very  minute 
quantity  of  a  radium  salt  spread  over  a  very  large 
area,  the  ft-  and  y-rays  from  so  small  a  quantity 
will  be  so  feeble  as  to  be  practically  negligible, 
whereas  the  a-rays  under  these  circumstances  will 
reach  their  greatest  intensity.  For  practical  pur- 
poses a  thin  film  of  pure  radium  salt  can  be  used 
to  give  a-rays  by  themselves,  essentially  free  from 
ft-  and  y-rays. 

Such  a  thin  film  I  have  prepared  for  these  ex- 
periments. On  this  shallow  platinum  dish,  about 
a  square  inch  in  area,  I  have  evaporated  down  a 
solution  containing  about  a  milligram  of  pure  radium 
bromide,  and  the  dish,  with  its  precious  film  open 
to  the  air,  is  carefully  preserved  when  not  in  use  in 
a  special  tube  containing  a  desiccating  agent  to 
keep  it  dry,  so  that  without  undue  risk  of  loss  I  can 
work  with  a  bare  film  of  radium  salt  and  show  you 
the  a-rays.  Over  the  bare  film  I  bring  the  electri- 
fied silk  tassel.  It  collapses  instantly,  in  fact,  much 
faster  than  it  does  when  brought  over  the  whole 
thirty  milligrams  of  radium  bromide  contained  in 
the  mica-covered  capsule.  The  a-rays  from  one 
milligram  of  radium  produce  more  electrical  effect 
than  the  ft-  and  y-rays  from  thirty  milligrams.  Now 
I  cover  the  bare  film  of  radium  with  a  single  sheet 
of  thin  writing-paper,  which  stops  the  a-rays  com- 
pletely, the  ft-  and  y-rays  scarcely  at  all.  You 


48  EXPERIMENTS    WITH  a-RAYS 

observe  the  tassel  remains  now  charged  as  if  the 
radium  were  absent.  The  /?-  and  y-rays  from  so 
small  a  quantity  hardly  appreciably  discharge  it. 

But  if  I  displace  the  paper  ever  so  slightly  and 
expose  a  tiny  part  of  the  bare  surface,  the  tassel 
instantly  collapses.  From  these  experiments,  and 
the  fact  that  it  was  the  fashion  at  the  time  to  cover 
radioactive  substances  when  experimenting  with 
them,  you  will  have  no  difficulty  in  understanding 
how  it  was  that  these  feebly  penetrating  but  in- 
tensely powerful  a-rays  remained  at  first  neglected 
and  almost  unknown. 

I  now  have  to  show  you  a  very  striking  experi- 
ment indeed,  suggested  by  some  profound  investi- 
gations of  Professor  Bragg  in  Adelaide,  on  the 
a-rays,  to  which  we  shall  again  have  occasion  to 
refer.  So  readily  are  these  a-rays  stopped  that  a 
few  inches  of  air  suffice  entirely  to  absorb  them. 
But  the  a-rays  show  this  remarkable  peculiarity  not 
exhibited  by  any  other  type  known.  Each  individual 
a-ray  of  any  one  homogeneous  type  travels  exactly 
the  same  distance  in  an  absorbing  medium,  and  is 
stopped  sharply  and  completely  when  a  certain 
thickness  of  matter  has  been  penetrated.  The  con- 
sequence is  that  if  we  work  with  a  homogeneous 
beam  of  a-rays,  just  without  the  distance  of  com- 
plete absorption,  there  is  absolutely  no  effect,  while 


FIG.  9.     Apparatus  to  show  Absorption  of  «-Rays  by  Air. 


FIG.  ii.     The  Spinthariscope  of  Sir  William  Crookes. 

To  face  p.  49. 


ABSORPTION  OF  0.-RAYS  BY  AIR 


49 


just  within  there  is  a  very  large  effect.  I  have  said 
that  the  a-rays  derived  from  radium  are  complex, 
consisting  of  four  different  types,  each  with  a 
definite  "  range,"  as  it  is  termed,  or  distance,  it  will 
travel  in  any  given  absorbing  medium.  For  the 


^PUMP 


FIG.  8. 


purposes  of  this  experiment,  however,  it  is  neces- 
sary to  consider  only  the  most  penetrating  type, 
which  Bragg  found  could  travel  in  air  at  atmo- 
spheric pressure  and  ordinary  temperature,  71 
millimeters  (or  just  under  three  inches)  and  no  more. 
Now  this  flask  (Figs.  8  and  9)  is  a  little  more  than 
six  inches  in  diameter,  and  it  has  been  coated  on  the 


50  ABSORPTION  OF  a-RA  YS  BY  AIR 

upper  hemisphere  of  the  inside  surface  with  a  phos- 
phorescent film  of  zinc  sulphide.  For  these  a-rays 
the  usual  phosphorescers  (e.g.  the  platinocyanides, 
willemite,  etc.),  employed  for  the  ft-  and  y-rays,  are  far 
less  sensitive  than  crystallised  zinc  sulphide,  or,  as  it 
is  called,  Sidot's  hexagonal  blende.  The  coated  flask 
is  arranged  so  that  I  can  plunge  my  platinum  dish 
with  its  bare  radium  film  upward  inside  the  flask 
and  hold  it  centrally  by  a  cork.  In  the  dark,  the 
flask  being  full  of  air,  you  observe  hardly  any 
glow.  The  three  inches  of  air  surrounding  the  radium 
film  on  all  sides  suffices  completely  to  stop  all  the 
a-rays,  and  the  ft-  and  y-rays,  from  so  small  a  quantity 
of  radium,  produce  only  a  negligible  effect  on  the 
zinc  sulphide.  But  I  have  connected  the  flask  to 
an  air-pump  and  can  pump  out  the  air.  At  the 
very  first  stroke  of  the  pump  the  whole  globe  flashes 
into  luminescence,  and  as  I  continue  pumping  the 
glow  gets  stronger  and  fairly  illuminates  the  im- 
mediate neighbourhood  with  its  soft  white  light. 
I  now  readmit  the  air,  and  the  glow  disappears  as 
suddenly  as  it  came.  So  that  you  see  with  some- 
what carefully  designed  arrangements,  and  keeping 
in  mind  the  peculiar  properties  of  these  a-rays 
which  physicists  have  exactly  worked  out,  it  is 
possible  even  from  a  minute  amount  of  pure  radium 
bromide  to  obtain  quite  a  fair  amount  of  light, 
whereas  the  same  quantity  of  radium  less  cunningly 


NATURE   OF  RADIATION  51 

disposed  would  give  very  little  effect.  Radium 
compounds  are  usually  preserved  in  sealed  tubes 
so  as  to  prevent  them  absorbing  moisture  from 
the  atmosphere.  Under  these  circumstances 
the  effects  produced  by  these  a-rays  are  not  ob- 
served. 

Problems  connected  with  the  real  physical  nature 
of  radiation  are,  it  is  well  recognised,  among  the 
most  fundamental  in  physics,  and  they  involve  more 
deeply  perhaps  than  any  others  the  great  under- 
lying metaphysical  relationships  between  the  external 
world  of  physical  fact  and  the  subjective  mental 
processes  by  which  we  attempt  to  visualise  these 
facts  and  obtain  some  sort  of  a  reasonable  explana- 
tion of  them.  Take,  for  example,  the  great  problem 
that  is  always  before  us  of  the  real  nature  of  light. 
Is  there  anything  more  difficult  of  mental  com- 
prehension? The  difficulties  are  not  minimised 
but  rather  increased  by  the  very  definite  view  we 
take  to-day  of  energy  as  a  separate  entity  having  a 
real  physical  existence. 

Contemplate  for  a  moment,  if  you  can,  the  origin 
of  the  energy  which  impels  every  moving  thing  in 
earth  or  sea  or  sky.  With  the  exception  of  a  very 
small  and  practically  negligible  movement  contri- 
buted by  the  tides  and  by  volcanic  agencies,  and,  it 
must  not  be  forgotten,  by  the  radioactive  substances 


52  THE  ETHER 

themselves,  all  things  which  move  do  so  directly 
or  indirectly  by  virtue  of  the  energy  reaching  this 
earth  as  radiations  in  the  form  of  the  sun's  light 
and  heat.  Great  masses  move  hither  and  thither 
here  because  of  happenings  at  some  time  past,  re- 
mote or  recent,  90  millions  of  miles  away  in  the 
sun.  Inevitably,  when  we  begin  to  contemplate 
radiation  phenomena,  we  are  driven  to  inquire  into 
the  medium  filling  the  outer  void  of  space  by  virtue 
of  which  this  immaterial,  but  vital  entity — energy — 
reaches  us  from  far  distant  worlds.  It  is  true  we 
call  it  ether^  and  try  to  give  to  it  all  sorts  of  material, 
or  pseudo-material,  characteristics.  Lord  Kelvin 
seems  to  have  spent  a  large  part  of  his  leisure  time 
trying  as  it  were  to  dematerialise  matter  into  ether, 
that  is,  trying  by  all  sorts  of  mechanically  ingenious 
arrangements  and  analogy  from  material  models— 
the  only  possible  models  our  minds  can  yet  grasp — 
to  obtain  a  possible  construction  which  would  simu- 
late the  elusive  but  all-pervading  ether.  Others,  on 
the  well-known  principle  that  topsy-turvydom,  if 
only  consistent  and  all-embracing  enough,  results 
finally  in  a  system  no  less  logical  and  rational  than 
the  original  one,  have  given  to  the  ether  inconceiv- 
ably great  density,  and  to  the  atoms  of  matter  the 
character  of  holes  or  voids  in  it.  The  necessity  for 
the  existence  of  a  universal  all-pervading  medium, 
or  ether,  capable  of  transmitting  energy,  no  one  in 


THE  ETHER  53 

these  days  of  wireless  telegraphy  would  deny,  but 
on  the  question  of  its  real  nature  opinion  is  as 
divided  as  it  well  could  be. 

The  tendency,  however,  in  modern  physics  to- 
day is  rather  to  derive  and  explain  material  pheno- 
mena from  the  properties  of  the  ether  than  to 
attempt  to  construct  an  ether  on  a  material  or 
pseudo-material  model.  As  yet,  however,  we  know 
little  about  the  properties  of  the  ether  itself.  One 
definite  thing  we  do  know,  for  certain,  and  have 
known  for  a  very  long  time,  namely  the  velocity 
at  which  influences  are  transmitted  across  the  ether. 
It  is  185,000  miles  a  second,  the  speed  of  light. 
So  far  as  we  yet  know,  all  influences  that  are 
transmitted  by  the  ether  travel  at  this  one  definite 
velocity.  Not  only  light,  but  also  the  electro- 
magnetic radiations  employed  in  wireless  tele- 
graphy, the  magnetic  storms,  as  they  are  termed, 
which  reach  us  from  the  sun,  and  also,  we  believe, 
the  X-rays,  travel  through  the  ether  at  this  one 
definite  speed. 

The  great  mind  of  Newton  two  centuries  ago 
appreciated  to  the  full  the  fundamental  difficulty 
in  the  explanation  of  radiation,  and  proposed  the 
only  way  of  escape  from  the  more  modern  doctrine 
of  an  ether  which,  so  far  as  I  know,  has  ever  been 
put  forward. 


54  CORPUSCULAR  RADIATION 

Light,  on  the  Newtonian  hypothesis,  consisted 
in  the  emission  from  the  glowing  body  of  exces- 
sively minute  material  particles  or  corpuscles  travel- 
ling with  immense  velocity.  This  corpuscular 
theory,  so  far  as  light  is  concerned,  failed  when 
subjected  to  a  closer  examination,  and  gave  way 
to  the  present  undulatory  theory  that  light  consists 
in  a  transverse  vibration  of  the  ether,  the  existence 
of  which,  it  was  beginning  to  be  recognised,  was 
as  great  a  necessity  for  the  transmission  of  gravita- 
tional, magnetic,  and  other  forms  of  energy  which 
reach  us  from  outer  space  as  it  was  for  the  trans- 
mission of  radiation  itself.  Though  proved  wrong 
so  far  as  light  is  concerned,  this  idea  of  corpuscular 
radiation,  strangely  enough,  will  rank  as  one  of 
the  most  suggestive  flashes  of  Newton's  genius,  for 
it,  in  fact,  anticipated  by  two  centuries  the  march 
of  experimental  discovery.  To-day,  thanks  to 
radioactivity,  science  has  been  enriched  by  the 
discovery  of  a-,  /3-,  and  y-rays,  and  two,  at  least, 
out  of  these  types,  the  a-  and  the  /3-rays,  are  not, 
like  light,  vibrations  of  the  ether,  but  consist  of  the 
emission  of  excessively  minute  material  particles 
(atoms  and  corpuscles)  travelling  with  immense 
velocity.  This  is  one  of  two  chief  main  lines 
of  evidence  that  radioactivity  is  an  accompanying 
manifestation  of  "  atomic  disintegration." 

Into  this  aspect  of  the  matter,  however,  I  do  not 


WAVE    THEORY  OF  LIGHT  55 

propose  entering  to-night.  Its  consideration  is 
more  conveniently  deferred.  It  is  sufficient  to  say 
that  the  a-  and  /3-rays,  or,  as  I  shall  henceforth  also 
refer  to  them,  a-  and  /3-particles,  comprise  the 
lighter  fragments,  as  it  were,  of  the  disintegrating 
atoms  of  the  radioactive  substance.  In  ordinary 
circumstances  radium  appears  to  be  expelling  both 
a-  and  /3-particles  together,  but  this  as  we  shall 
come  to  see  is  due  to  the  fact  that  several  successive 
disintegrations  are  occurring,  and  the  effect  is  a 
composite  one.  The  nature  of  <these  rays  is  so 
utterly  different  from  that  of  light  that  it  is  worth 
while  to  stop  and  examine  the  difference  a  little 
more  closely. 

The  wave  theory  of  light  has  often  been  illus- 
trated by  what  happens  when  a  stone  is  dropped 
into  a  pool.  Ripples  extend  outwards  in  concentric 
circles  from  the  disturbance.  The  water,  as  the 
ripple  reaches  it,  first  rises  above,  then  immediately 
afterwards  falls  below  the  normal  level.  The  dis- 
turbance is  propagated  transversely,  that  is,  out- 
wards horizontally  by  a  vertical,  or  up  and  down 
wave-movement  of  the  water.  The  surface  dis- 
closes the  nature  of  the  disturbance,  but  the  same 
type  of  disturbance  is  taking  place  below  the 
surface,  and  each  circular  ripple  is  in  reality  the 
section  of  a  hemispherical  shell.  It  is  not  possible 


56      DISCRETE    THEORY  OF  RADIUM  RAYS 

to  get  an  ether  surface  like  a  water  surface,  since 
the  ether  is  all-pervading.  Light  travels  out  from 
an  incandescent  point  in  all  directions  in  spherical 
ripples,  in  which  a  to-and-fro  motion  of  some  kind 
is  going  on  in  the  ether,  transverse  to  the  direction 
of  propagation  of  the  light.  Contrast  with  this  what 
is  believed  to  be  the  nature  of  the  a-  and  #-rays 
given  out  from  a  radioactive  substance.  The  rays 
are  given  out  uniformly  in  all  directions,  not  as  a 
succession  of  spherical  waves,  but  as  the  random 
flight  of  immense  swarms  of  tiny  projectiles  ejected 
from  the  radioactive  substance.  For  shortness  I 
shall  call  this  the  "discrete  theory,"  as  contrasted 
with  the  wave  theory,  because  the  radiation  is 
considered  to  be  due  to  the  flight,  radially  outward 
from  the  substances  like  the  spokes  of  a  wheel,  of 
swarms  of  free-flying,  independent  discrete  particles. 
You  could  hardly  imagine  two  more  different  pheno- 
mena, and  yet  that  it  is  not  easy  to  distinguish 
between  their  effects  is  shown  by  the  fact  that 
for  a  long  time  a  controversy  raged  between  the 
two  views  regarding  the  nature  of  light  itself. 

I  must  anticipate  a  little  here  for  the  sake  of 
clearness.  It  is  now  an  old  story  that  in  the  tiniest 
grain  of  matter  there  is  a  mentally  inconceivable 
myriad  of  separate  atoms.  In  this  tiny  quantity 
of  radium  bromide,  weighing  half  a  grain,  we  know 
with  fair  certainty  there  are  fifty  million  billion 


DISCRETE    THEORY  OF  RADIUM  RAYS      57 

(5  x  io19)  separate  atoms  of  radium,  assuming  that 
the  compound  is  pure.  It  has  been  proved  that, 
roughly,  one  two-thousandth  of  these  disintegrate 
yearly.  There  are  about  32,000,000  seconds  in  a 
year,  so  that  in  every  second  of  time  rather  less  than 
one  thousand  million  of  these  radium  atoms  dis- 
integrate, giving  some  small  multiple  of  this  number 
of  a-  and  /3-particles.  So  mighty  a  host  projected 
outwards  in  all  directions  at  random,  as  you  may 
suppose,  fill  the  surrounding  space  with  their 
trajectories  to  all  intents  and  purposes  as  com- 
pletely as  if  they  advanced  as  one  continuous 
spherical  wave-front.  In  other  words,  if  only  the 
number  of  projected  particles  is  sufficiently  great 
a  discrete  radiation  will  be,  in  many  of  its  general 
effects  and  laws  of  propagation,  not  different  from 
a  wave-radiation.  It  is  true  that  such  a  radiation 
will  show  neither  regular  reflection,  refraction,  nor 
polarisation  in  the  manner  that  light  does,  and  the 
absence  of  these  phenomena  for  the  a-  and  /3-rays 
is  part  of  the  evidence  in  favour  of  their  discrete 
nature.  If,  however,  we  continuously  reduce  the 
number  of  particles  ejected,  in  other  words,  if  we 
continuously  diminish  the  quantity  of  radium  em- 
ployed, there  should  come  a  point  when  the 
discrete  radiation  should  no  longer  simulate  the 
wave-type.  It  should,  as  it  were,  break  up  and 
show  discontinuity,  much  as  some  of  those  faint 


58  RESOLUTION  OF  a-RAYS 

continuous  light-patches  in  the  heavens,  known  as 
the  planetary  nebulae,  when  investigated  by  more 
and  more  powerful  telescopes,  begin  to  break  up  and 
show  discontinuity,  and  finally  are  resolved  into  an 
innumerable  host  of  separate  twinkling  stars.  Is 
it  possible  so  to  resolve  a  swarm  of  a-rays  ? 

The  older  physicists  who  first  deduced  by  accu- 
rate computation  the  weight  and  measure  of  the 
single  individual  atom  and  evaluated  the  number  of 
billions  contained  in  the  smallest  portion  of  matter 
perceptible  to  the  senses,  had  they  been  soberly 
asked  whether  it  would  be  possible  ever  to  observe 
a  single  atom  of  matter,  would  have  scouted  the 
bare  possibility.  A  single  atom  of  matter ! — A 
single  atom  of  matter  ! —  I  recall  this  one  exclama- 
tion, repeated  over  and  over  again  with  varying 
intonation  by  a  distinguished  foreign  visitor,  whose 
years  had  been  spent  at  the  microscope  on  the  border- 
land between  the  perceptible  and  the  imperceptible 
worlds,  when  the  question  we  are  now  considering 
was  under  discussion  at  a  British  Association  meeting. 

Let  us,  however,  now  make  a  few  calculations  to 
see  whether  there  is  any  hope  whatever  of  being 
able  to  detect  the  effect  of,  say,  a  single  a-particle 
expelled  from  radium,  in  the  same  sense  as  it  has 
been  found  possible  in  astronomy  to  detect  the  indi- 
vidual stars  which  go  to  make  up  a  planetary  nebula. 


THE   SPINTHARISCOPE  59 

In  my  first  lecture  (p.  23)  I  alluded  to  the 
smallest  quantity  of  radium  that  could  be  detected 
by  the  aid  of  the  gold-leaf  electroscope,  that  is, 
therefore,  by  means  of  the  a-rays  emitted.  It  was 
one  three-thousand-millionth  of  a  grain.  Half  a 
grain,  as  we  have  seen,  gives  out  a  few  thousand 
million  a-particles  every  second.  So  that  the 
smallest  quantity  of  radium  detectable  by  the 
ordinary  electroscope  must  be  giving  out  only  a  few 
individual  a-particles  per  second.  From  a  very 
early  stage  it  appeared  not  inconceivable  to  Ruther- 
ford that  a  discontinuity  in  the  emission  of  a-rays 
might  actually  be  detected  by  using  a  very  minute 
quantity  of  radium. 

The  Spinthariscope: — The  problem  was  actually 
solved,  almost  unawares,  by  Sir  William  Crookes, 
by  means  of  an  instrument  he  devised  and  called 
the  Spinthariscope.  The  instrument  is  the  only 
genuine  instrument  worked  by  radium  that  it  is 
at  present  possible  to  buy  at  the  optician's  in  the 
ordinary  way,  and  it  can  be  bought — radium  and 
all — for  a  few  shillings.  The  reason  for  this 
apparent  paradox  is  to  be  found  in  the  fact  that 
it  is  in  the  essence  of  the  result  to  be  attained  to 
reduce  the  amount  of  radium  to  the  smallest  possible 
quantity,  and  this  unusual  condition  allows  of  a 
practically  unlimited  number  of  spinthariscopes  to 


6o 


THE  SPINTHARISCOPE 


be  made  out  of  an  almost  invisible  quantity  of 
radium  bromide.  The  amount  of  radium  in  each 
instrument  is  absolutely  unweighable  and  invisible. 
A  needle,  A,  is  made  to  touch  a  tiny  phial  which  once 
contained  radium,  and  is  then  mounted  (Figs.  10  and 
i  T,  opposite  p.  49)  centrally  in  a  little  brass  tube,  the 


FIG.  10. 

size  of  a  small  reel  of  cotton,  at  the  bottom  of  which  is 
a  phosphorescent  screen,  B,  coated  with  zinc  sulphide. 
At  the  other  end  of  the  tube  is  a  lens,  C,  for  magnify- 
ing the  screen  and,  by  means  of  a  little  screw,  D,  out- 
side, the  needle  point  may  be  moved  nearer  to  or  away 
from  the  screen.  If  now  in  a  dark  room  the  screen 
is  observed  through  the  lens,  it  will  be  seen  to  be 
luminous,  and  this  luminosity  can  be  concentrated 


THE   SPINTHARISCOPE  61 

or  spread  out  by  screwing  the  needle  point  nearer 
to  or  farther  from  the  screen.  After  the  eye  has 
become  used  to  the  darkness  it  will  be  seen  that  the 
luminosity  is  not  just  a  quiet  continuous  glow.  The 
light,  like  that  of  the  planetary  nebulae,  has  been 
resolved  and  shows  discontinuity.  It  resembles 
most  nearly  a  shower  of  shooting  stars.  Bright 
momentary  flashes  of  light  or  scintillations,  too 
numerous  at  any  instant  to  count,  are  appearing  and 
disappearing  in  the  field  of  vision.  These  flashes 
are  caused  by  the  a-particles  of  radium.  This 
minute  insignificant  trace  of  radium  is  positively 
belching  forth  a-particles.  It  seems  incredible  that 
the  incessant  bombardment  of  the  screen  can  be 
caused  by  such  an  infinitesimal  amount  of  radium. 
Yet  so  it  is,  and  in  a  month's  time,  if  the  instrument 
is  re-examined,  it  will  be  found  that  the  scintillations 
are  as  numerous  and  as  brilliant  as  formerly. 
After  a  time,  perhaps  a  year,  the  phosphorescent 
screen  itself  will  be  worn  out  by  the  incessant 
bombardment,  will  become  insensitive  and  need 
renewal.  But  replace  it  by  a  new  one  and  the 
radium  will  be  found  to  be  as  energetic  as  ever. 
The  owner  of  the  instrument  will  pass  away,  his 
heirs  and  successors,  and  even  his  race  will  probably 
have  been  forgotten  before  the  radium  shows  any 
appreciable  sign  of  exhaustion. 


62  THE  INDIVIDUAL  a- PARTICLE 

The  actual  a-particle  itself  must,  of  course,  be 
extremely  small.  How  else  could  a  mere  speck 
of  radium  send  out  such  an  incessant  and 
numerous  swarm  ?  As  we  have  still  to  prove, 
the  a-particle  is  an  atom  of  helium,  the  second 
lightest  atom  of  matter  known.  A  grain  of  radium 
bromide  expels  every  second  about  ten  thousand 
million  a-particles,  and  if  we  contemplate  this 
mighty  swarm  expelled  once  every  second  of  time 
throughout  many  centuries  we  may  begin  to  have 
some  idea  of  how  many  atoms  there  must  be  in  a 
single  grain  of  matter,  and  how  small  must  be  the 
single  atom.  The  philosophers  of  only  a  decade 
ago  would  have  ridiculed  the  hope  that  we  should 
ever  be  able  to  look  through  a  magnifying-glass  to 
see  the  effect  of  a  single  atom  of  matter,  yet  each  of 
the  scintillations  of  the  spinthariscope  is  nothing 
else. 

The  spinthariscope  was  the  original,  but  to-day 
it  is  only  one  of  many  lines  of  evidence  which 
have  established  the  discrete  character  of  the 
a-radiation  and  the  nature  of  the  a-particle.  We 
know  of  many  radioactive  substances — polonium 
is  one — emitting  a-radiattons,  which  gradually  and 
completely  lose  their  radioactivity  with  the  lapse 
of  time.  Anticipating,  we  may  say  that  the  dis- 
integration of  polonium  proceeds  so  rapidly  that  it 


THE  INDIVIDUAL   o.-P ARTICLE  63 

is  complete  in  the  course  of  a  few  years.  Were  the 
process  at  all  similar,  for  example,  to  the  case  of 
a  hot  body  cooling,  one  would  expect  a  gradual 
alteration  in  the  character  of  the  radiation  with  the 
diminution  of  its  intensity  with  lapse  of  time ; 
whereas  the  character  of  the  radiation  is  exactly 
the  same  at  the  end,  when  it  has  nearly  all  decayed, 
as  it  is  at  the  beginning.  This  is  explained  simply 
on  the  view  that  the  number  of  a-particles  expelled 
grows  less  as  the  activity  decays.  The  individual 
a-particles  have  the  same  velocity  and  other  charac- 
teristics, whether  expelled  at  the  end  or  at  the 
beginning  of  the  process.  Professor  Bragg's  dis- 
covery that  each  a-particle  has  a  definite  "range," 
characteristic  of  it,  is  quite  inexplicable  on  a  wave 
theory.  The  range  of  the  a-particles  emitted  by 
polonium,  for  example,  is  thirty-eight  millimetres  of 
air,  and  though  in  the  course  of  a  few  years  the 
a-radiation  of  polonium  decays  always  completely, 
the  range  of  the  a-particle  expelled  at  the  end  is 
exactly  the  same  as  at  the  beginning. 

In  this  connection,  finally,  I  may  mention  some 
really  wonderful  work  recently  done  by  Professor 
Rutherford  and  his  co-worker  Dr.  Geiger,  in  which 
they  have  actually  succeeded  in  counting  directly 
the  number  of  a-particles  expelled  from  a  given 
quantity  of  radium  every  second.  As  you  may 


64  COUNTING    THE  ^-PARTICLES 

know,  if  two  points  are  connected  to  an  electrical 
machine,  or  other  method  of  generating  an  electric 
force  or  tension,  a  spark  will  pass  between  them 
under  suitable  circumstances.  Now  suppose  the 
distance  apart  of  the  two  points  is  just  so  great 
that  no  spark  will  pass  with  the  particular  electrical 
tension  applied,  and  that  some  radium  is  then 
brought  near  to  the  points.  Then  a  spark  will 
pass.  The  rays  from  radium  by  making  the  air 
a  conductor  of  electricity  facilitate  the  passage  of 
the  spark,  so  that  under  their  influence  the  discharge 
will  leap  across  a  greater  distance  than  it  otherwise 
would.  Substitute  for  the  crude  method  of  detect- 
ing the  discharge  by  means  of  a  spark  a  highly 
refined  electrical  instrument,  known  as  the  electro- 
meter, in  which,  as  in  the  galvanometer,  a  spot  of 
light  is  reflected  from  a  mirror  attached  to  a  needle, 
which  can  be  arranged  to  move  when  a  discharge 
passes  across  the  gap,  and  you  have  the  essential 
principle  of  Rutherford's  arrangement.  Such  an 
arrangement  can  be  made  so  excessively  sensitive 
that  the  passage  of  a  single  a-particle  from  radium 
through  what  corresponded  to  the  " spark  gap" 
of  the  first  arrangement  described,  is  sufficient 
to  cause  the  spot  of  light  from  the  needle  of  the 
electrometer  to  move  with  a  sudden  jerk.  The 
experiment  consists,  then,  in  counting  the  number 
of  these  sudden  jerks  of  the  electrometer  needle  in 


COUNTING    THE   ^-PARTICLES  65 

a  given  time,  when  a  known  quantity  of  radium  is 
placed  at  a  known  distance.  The  radium  has  to  be 
placed  many  yards  away  from  the  apparatus,  and 
the  a-rays  are  fired  along  a  long  exhausted  tube 
with  a  small  window  at  the  end  to  admit  the 
passage  of  a  very  minute  definite  proportion  of  the 
total  number  of  a-particles,  which  proportion  can  be 
calculated.  In  the  actual  experiments  the  distance 
of  the  radium  and  the  size  of  the  window  through 
which  the  a-particles  passed  were  such  that,  roughly, 
only  one  out  of  every  100  million  a-particles  ex- 
pelled found  their  way  into  the  apparatus.  The 
total  number  of  a-particles  actually  expelled  per 
second  by  a  grain  of  radium  in  its  normal  condition 
was  found  to  be  about  ten  thousand  million.  Per 
milligram  of  radium  the  exact  number  per  second 
is  136  million.  These  results  were  also  checked  by 
counting  the  number  of  scintillations  per  second  in 
a  special  form  of  spinthariscope.  There  have 
always  been  scientific  men  who  have  regarded  the 
atom  and  the  atomic  theory  with  suspicion  and 
have  never  tired  of  insisting  upon  its  "  hypo- 
thetical "  character.  It  may  therefore  be  rightly 
regarded  as  one  of  the  greatest  triumphs  of  science 
that  an  observer  can  now  actually  sit  down  in  front 
of  a  vessel  and  with  the  aid  of  a  watch  count  the 
number  of  atoms  entering  it  every  minute  from  a 
quantity  of  radium  outside. 


CHAPTER  IV. 

The  £-rays — Their  deviability  by  a  magnet — The  nature  of  the 
/3-particle— Analogy  to  cathode-rays  or  "  Radiant  Matter  " — The 
electron — Velocity  of  the  /3-rays — The  nature  of  the  a-particle — 
Its  velocity — Its  power  of  passing  through  atoms  of  matter  in  its 
path— Scattering  of  a-particles— Method  of  rendering  the  track  of 
rays  visible— a-Particles  might  be  expelled  without  their  being 
detectable. 

IN  addition  to  their  varying  power  of  penetrating 
matter,  there  is  another  test  which  has  proved 
of  great  service  in  analysing  the  three  types  of  rays 
from  radioactive  bodies  and  in  determining  the  real 
nature  of  each.  The  trajectories  of  some  of  the 
rays  are  powerfully  influenced  by  a  magnet,  while 
others  are  hardly  if  at  all  affected.  Thus  the  /3-rays 
of  all  radioactive  substances  if  caused  to  traverse 
the  space  between  the  poles  of  a  magnet  are  very 
strongly  deflected,  and  if  the  magnet  is  a  power- 
ful one  may  be  completely  coiled  up  into  closed 
circles  or  spirals. 

Faraday  imagined  that  between  the  N-pole  and 
S-pole  of  a  magnet  there  existed  actual  lines  of 
magnetic  force.  In  the  electro-magnet  on  the  table 
(Fig.  12),  which  is  formed  so  that  the  N-  and 
S-poles  are  bent  round  so  as  to  face  one  another, 
the  lines  of  force  between  the  opposite  faces  of  the 

66 


I  4 


To  face  p.  66. 


THE  ELECTRO-MAGNET  67 

two  pole-pieces  are  straight  lines  following  the 
shortest  distance  between  them.  It  is  convenient  to 
imagine  with  Faraday  the  actual  existence  of  such 
lines  of  force.  An  electro-magnet  is  simply  an 
arrangement  in  which  a  bar  of  soft  iron  can  be 
magnetised  at  will  by  passing  an  electric  current 
through  a  coil  of  wire  wound  round  it.  Soft  iron 
of  good  quality,  unlike  steel,  retains  no  appreciable 
permanent  magnetism.  It  is  very  easily  magnetised 
by  an  electric  current,  and  its  magnetism  continues 
just  so  long  as  the  current,  and  ceases  practically 
completely  when  the  current  is  switched  off. 

Now  suppose  a  beam  of  /3-rays  of  radium  to 
be  fired  through  the  space  between  the  pole-pieces 
at  right  angles  to  the  lines  of  magnetic  force.  The 
path  of  the  rays  is  bent.  The  rays  tend  to  coil 
round  the  magnetic  lines  of  force  in  circles.  Suppose 
we  look  along  the  lines  of  force  stretching  from  the 
N-pole  to  the  S-pole,  that  is  to  say,  suppose  the 
eye  to  be  placed  at  the  centre  of  the  N-pole  and 
to  be  looking  towards  the  centre  of  the  S-pole. 
Then  the  /3-rays  will  be  coiled  round  into  circles 
in  a  direction  of  rotation  opposite  to  that  of  the 
hands  of  a  clock,  that  is,  as  we  say,  counter  clock- 
wise. If  we  look  from  the  S-pole  to  the  N-pole 
the  direction  of  rotation  is  clock-wise.  Now  if 
the  radium  is  placed  behind  the  poles  of  the  electro- 
magnet, and  a  screen  of  platinocyanide  of  barium 


68 


MAGNETIC  DEVIATION  OF  $-RAYS 


is  placed  in  front,  and  the  distance  between  them 
is  so  adjusted  to  the  strength  of  the  magnet  that 
when  the  latter  is  excited  by  an  electric  current  the 


SCREEN 


Magnet  off. 


SCREEN 


Magnet  on. 
FIG.  13. 

/3-rays  from  the  radium  are  coiled  up  into  circles  of 
lesser  diameter  than  the  distance  between  the  radium 
and  the  screen,  none  of  the  /3-rays  will  now  reach 
the  screen.  This  will  be  seen  from  Fig.  13.  In 
this  figure  the  eye  is  supposed  to  be  at  the  centre 
of  the  S-pole  of  the  magnet,  looking  towards  the 


MAGNETIC  DEVIATION  OF  $-RAYS  69 

face  of  the  N-pole.  The  rays  from  the  radium 
passing  up  between  the  N-pole  and  the  eye,  in  the 
top  diagram,  reach  the  screen.  In  the  lower  dia- 
gram the  magnet  is  in  action,  and  the  rays  are  coiled 
clock-wise  into  circles,  none  reaching  the  screen. 

The  radium  is  contained  in  its  mica-covered 
capsule  so  that  only  the  ft  and  y-rays  are  dealt 
with,  the  a-rays  being  suppressed.  In  the  darkness 
you  see  the  phosphorescent  screen  brilliantly  lumi- 
nous so  long  as  the  magnet  is  not  excited.  I 
switch  on  the  current  and  the  light  of  the  screen 
at  once  goes  out  almost  completely.  The  faint 
luminosity  left  behind  is  due  to  the  y-rays,  which 
are  not  deviated  at  all,  so  far  as  we  know,  even  by 
the  strongest  magnetic  forces.  If  I  interpose  a 
penny  in  front  of  the  radium  so  that  the  y-rays 
have  now  to  traverse  it  before  reaching  the  screen 
the  faint  luminosity  is  hardly  diminished.  Now  I 
switch  off  the  exciting  current  and  the  magnet 
almost  instantly  loses  its  magnetism,  the  /3-rays 
spring  back  out  of  their  circular  into  straight  trajec- 
tories, strike  the  screen  and  cause  it  again  to  flash 
out  into  brilliance.  Now  the  introduction  of  a 
penny  causes  the  luminosity  practically  to  disappear, 
all  but  for  the  faint  glow  due  to  the  y-rays. 

To  a  trained  physicist  the  interest  of  this  be- 
haviour is  due  to  the  fact  that  it  is  exactly  what 


7o  ACTION  OF  MAGNETISM 

would  happen  to  a  current  of  electricity  if  it  were 
made  to  flow  between  the  poles  of  a  strong  electro- 
magnet. If  we  employed  a  piece  of  ordinary  wire 
to  carry  the  current,  the  wire  would  tend  to  coil 
up  into  a  circle  exactly  like  the  /3-ray,  and  there 
would  be  a  battle  between  the  natural  stiffness  of 
the  wire  and  the  deviating  magnetic  force,  and  it 
would  depend  on  their  relative  strengths  which 
prevailed.  With  care,  however,  it  is  possible  to 
use  a  fluid  wire,  which  has  no  stiffness.  If  a  strong 
current  is  passed  through  a  thin  aluminium  wire  it, 
of  course,  gets  hot  and  finally  melts,  but  retains  its 
original  form  without  breaking,  hanging  by  virtue 
of  its  weight  as  a  beautiful  loop  of  glowing  molten 
aluminium.  Such  a  loop  provides  an  extremely 
sensitive  means  of  investigating  the  laws  of  action 
of  magnets  on  currents,  and  you  can  see  how 
violently  and  powerfully  it  is  deviated  if  it  is  hung 
between  the  poles  of  the  electro-magnet  and  the 
magnet  then  excited.  The  /3-rays,  as  they  traverse 
their  course,  behave  exactly  like  a  current  of  elec- 
tricity. If  they  consisted  of  extremely  rapidly 
moving  particles — charged  with  electricity — we  know 
that  such  a  stream  would  behave  to  a  magnet  ex- 
actly like  a  current  flowing  in  a  flexible  conductor. 

Now  we  do   know   the  direction   in    which  the 
/3-rays  are  moving,    namely  from  the  radium,   but 


UPON  THE  ELECTRIC   CURRENT  71 

we  do  not  know,  or  at  least  did  not  till  recently 
know,  the  direction  in  which  the  electricity  is  mov- 
ing in  an  electric  current.  However,  by  long  usage 
we  speak  in  a  purely  conventional  way  of  the  + 
and  -  ends  of  a  wire  in  which  a  current  is  flowing. 
We  do  not  yet  know  for  certain  whether  there  are 
two  kinds  of  electricity,  a  positive  kind  and  a  nega- 
tive kind,  or  whether  there  is  only  one  kind,  the 
effects  of  the  opposite  kind  being  due  to  a  relative 
electrical  scarcity  or  vacuum.  It  is  much  the  same 
with  heat  and  cold,  except  that  we  know  the  real 
thing  is  heat  and  cold  is  the  absence  of  it.  A 
trained  physicist  will  speak  of  so  much  heat,  or  so 
little  heat,  or  of  one  body  having  so  much  less  heat 
than  another,  but  he  will  not  speak  of  so  much  cold, 
or  one  body  having  more  cold  than  another, 
although  often  such  a  method  of  expression  would 
be  convenient  and  would  lead  to  no  error.  In  this 
sense  we  may  speak  both  of  positive  and  negative 
electricity  without  error.  A  current  of  electricity 
flowing  along  a  wire  from  the  positive  to  the 
negative  we  may  look  upon  as  due  to  the  transport 
of  positive  electricity  in  the  direction  from  +  to  - , 
or  as  the  transport  of  negative  electricity  from  -  to 
+  .  The  two  ideas  are  equivalent  and,  in  fact, 
identical  for  the  present  purposes.  On  the  view 
that  there  is  only  one  kind — the  negative  kind — of 
electricity,  a  positively  charged  body  or  atom  is 


72  ELECTRIC   CHARGE    OF  ^-R AYS 

merely  a  body  or  atom  with  less  negative  electricity 
than  is  normally  present  in  an  "  uncharged "  or 
electrically  neutral  body. 

In  the  /3-rays  we  have  a  movement  of  charged 
particles  from  the  radium,  and  we  have  to  find  out 
whether  the  particles  are  positively  or  negatively 
charged,  using  the  terms  positive  and  negative  in 
their  conventional  electrical  significance.  If  the 
rays  were  deviated  in  the  same  sense  as  a  current 
flowing  from  +  to  -  in  the  same  direction  as  the 
rays,  obviously  we  should  conclude  the  /3-rays 
were  +  ly  charged.  As  a  matter  of  fact,  we  find 
the  opposite  is  the  case.  When  the  /3-rays  are 
deviated  clock-wise  by  the  magnet  a  current  of  the 
kind  described  would  be  deviated  counter  clock- 
wise. To  simulate  the  deviation  of  the  /3-rays  the 
electric  current  must  be  a  negative  current,  that  is 
to  say,  must  be  either  negative  electricity  flowing  in 
the  direction  of  the  rays,  or  positive  electricity 
flowing  in  the  opposite  direction.  As  there  is  no 
reason  to  doubt  that  the  /3-rays  do  come  from  the 
radium,  the  electric  charge  they  carry  must  be 
negative. 

Modern  views  are  definite  on  the  point  that  if 
there  is  only  one  electricity,  that  one  is  the  kind 
which  by  convention  has,  unfortunately,  been 
styled  negative.  The  negative  is  the  real  electri- 
city. The  positive  may,  like  cold,  be  the  mere 


ANALOGY  TO   CATHODE-RAYS  73 

deficit  of  the  real  kind,  or  it  may  have  a  separate 
existence,  the  mirror  image  as  it  were  of  the  other 
kind.  I  personally  prefer  the  view  that  negative 
electricity  is  " electrical  heat"  and  positive  electri- 
city "  electrical  cold,"  but  a  real  answer  to  this 
question  would  no  doubt  prove  itself  to  be  a  very 
fundamental  step,  and  may  not  be  forthcoming  for 
a  long  time. 

The  behaviour  of  the  /3-rays  in  a  magnetic  field 
associates  them  at  once  with  some  previously 
known  radiations  from  the  electrical  discharge 
tubes  exhausted  to  an  extremely  high  degree  of 
vacuum  which  are  known  generically  as  Crookes' 
tubes,  from  their  first  systematic  investigator.  Into 
this  field  of  work  I  have  no  intention  of  entering 
in  detail,  for  it  is  the  one  aspect  of  this  subject 
which  has  received  the  most  adequate  treatment  in 
the  accounts  of  radioactivity  written  for  the  benefit 
of  the  public.  A  brief  resume  only  must  suffice. 

The  /3-rays  are  very  similar  in  nature  to  the 
"  Radiant  Matter"  (also  called  "  cathode-rays  "  or 
"  cathode-streams ")  of  Sir  William  Crookes,  ob- 
tained when  an  electric  discharge  or  current  is 
passed  through  a  vessel  exhausted  to  a  very  high 
degree  of  vacuum.  The  requisite  degree  of 
vacuum  can  be  obtained  with  a  little  trouble  by 
the  aid  of  a  mercury  pump,  based  on  the  same 


74  THE    CROOKED    TUBE 

principle  as  Toricelli's  celebrated  experiment  with 
the  barometer.  But  far  quicker  and  more  efficient 
methods  have  lately  come  into  use.  One  such 
consists  in  absorbing  the  last  traces  of  gas  in  the 
pores  of  the  charcoal  of  cocoa-nuts  cooled  to  the 
temperature  of  liquid  air,  according  to  the  dis- 
covery of  Sir  James  Dewar.  Another  method 
consists  in  absorbing  the  last  traces  of  gas  with 
the  vapour  of  metallic  calcium  heated  to  a  very 
high  temperature  in  a  special  vacuum  furnace. 
The  discharge  round  the  cathode,  or  negative  pole, 
in  a  high  vacuum  then  consists  of  radiant  streams 
of  particles  travelling  in  straight  lines  and  pro- 
ducing vivid  green  phosphorescence  where  they 
strike  the  glass  walls  of  the  vessel.  Any  obstacle 
placed  in  their  path  casts  a  sharp  shadow,  the  glass 
beyond  not  fluorescing  where  protected  from  the 
bombardment  by  the  obstacle.  These  particles 
also  carry  charges  of  negative  electricity,  and  have 
great  energy,  heating  to  whiteness  a  piece  of 
platinum  interposed  in  their  path,  and  causing  the 
most  intense  fluorescence  of  willemite  in  the  same 
way  as  the  radium  rays.  Like  the  /3-rays  they  are 
deviated  by  a  magnet,  and  in  the  same  sense,  only 
very  much  more  easily.  Here  is  a  form  (Fig.  14) 
of  Crookes'  tube,  designed  to  show  the  cathode- 
rays  and  their  deviation  by  the  action  of  a  magnet. 


CD 


FIG.  14 


76  DEVIATION  OF  CATHODE-RAYS 

The  electrodes  consist  of  plates  of  metal,  which  are 
attached  to  the  terminals  of  an  induction  coil,  or  an 
electrical  machine  or  other  sufficiently  powerful 
source  of  electric  tension.  One  electrode  A  is  con- 
nected to  the  positive  pole,  and  the  other  electrode  B 
to  the  negative  pole  of  the  coil,  and  we  have  to  con- 
centrate our  attention  on  the  negative  electrode,  this 
being  what  is  also  called  the  "cathode."  The 
tube  has  been  exhausted  by  a  pump  until  there 
is  only  about  one  ten-thousandth  of  the  air  left, 
and  was  then  sealed  up.  Under  such  conditions 
the  glass  of  the  tube  shines  with  a  brilliant  fluores- 
cence when  a  discharge  is  forced  through  it.  This 
fluorescence  has  been  traced  to  "  rays  "  inside  the 
vessel,  proceeding  from  the  cathode  at  right  angles 
to  its  surface  and  travelling  in  straight  lines 
through  the  tube.  Wherever  they  strike  the  glass 
they  cause  it  to  glow,  just  as  the  radium  rays  do. 

In  front  of  the  cathode  is  a  piece  of  mica  with 
a  slit  cut  in  it,  which  stops  all  the  rays  except  a 
narrow  pencil  passing  through  the  slit.  Along  the 
length  of  the  tube  is  fixed  a  fluorescent  screen  in  the 
form  of  a  plate  painted  with  powdered  willemite, 
and  as  the  narrow  pencil  of  rays  impinge  on  this 
plate  they  trace  out  their  path  as  a  bright  line  of 
green  fluorescence.  Now  if  one  pole  of  a  magnet 
is  brought  behind  the  tube  the  rays  are  bent 
sharply  to  the  left  or  right,  depending  on  whether 


THE  ELECTRON  77 

the  N-  or  the  S-pole  of  the  magnet  is  presented  to 
the  tube.  The  direction  of  the  deviation  is  the 
same  as  with  the  /3-rays,  and  before  even  the 
/3-rays  had  been  discovered  the  cathode-rays  of  the 
Crookes'  tube  had  been  definitely  shown  to  consist 
of  minute  particles  charged  with  negative  electricity 
flying  off  from  the  cathode  with  immense  velocity. 

What  are  these  particles  ?  Crookes  thought  they 
were  matter  in  a  new  or  fourth  state.  To-day 
we  know  they  are  "electrons."  The  electron  is  a 
new  and  somewhat  startling  conception  to  minds 
trained  on  the  older  lines,  although  traces  of  it 
date  back  from  the  discoveries  of  Faraday  of  the 
laws  of  electrolysis.  The  electron  is  an  atom  of 
electricity,  divorced  from  matter,  and  the  cathode- 
rays  consist  of  these  separate  individual  and  iso- 
lated electrons,  repelled  out  of  the  metal  of  the 
negative  pole  under  the  action  of  powerful  electric 
stress,  and,  in  the  absence  of  gas,  gathering  terrific 
speed  in  their  passage  through  the  exhausted  tube. 
Whatever  the  manner  in  which  these  electrons  are 
produced,  under  whatever  circumstances  they  result, 
they  are  always  identical  in  their  main  character- 
istics. Their  charge  is  always  the  same,  and  also 
their  "mass,"  although  their  velocity  may  and  does 
vary  according  to  the  conditions  within  very  wide 
limits.  They  and  their  motion  are  responsible  for 


78  "MASS"    OF  THE   ELECTRON 

the  most  varied  and  apparently  unconnected 
phenomena  in  nature,  and  in  the  empire  of  matter 
they  seem  often  to  occupy  a  role  in  comparison  with 
the  more  massive  material  atoms  analogous  to  the 
part  played  by  the  planets  in  relation  to  the  central 
sun  of  a  solar  system.  The  mass  of  the  electron 
is  only  one  thousandth  part  of  that  of  the  hydrogen 
atom,  the  smallest  particle  previously  known. 

In  some  ways  we  know  far  more  about  the 
electron  than  about  the  atom  of  matter.  The  elec- 
tron cannot  move  without  disturbing  the  medium 
which  occupies  all  space  continuously,  and  which 
we,  not  yet  knowing  too  much  about  its  real  nature, 
call  the  ether.  It  is  the  motion  and  change  of 
motion  of  the  electron  which  give  us  light,  the 
X-rays,  and  the  long  ether  waves  used  in  wireless 
telegraphy.  It  is  the  reaction  of  the  ether  on  the 
moving  electron  which  gives  it  its  "  mass."  Now  this 
1  'mass"  of  the  electron,  applied  as  the  term  was  to 
the  atom  of  pure  electricity  entirely  unassociated  with 
matter,  needed  very  careful  and  clear  thinking,  or 
it  would  appear  utterly  contradictory  to  the  older 
conceptions  of  matter.  The  term  mass,  used  in 
this  sense,  has  nothing  to  do  with  the  effect  of 
gravity  or  weight,  as  it  is  still  absolutely  unknown 
whether  electrons  obey  the  law  of  gravitation. 

In  this  region  of  new  ideas  we  are  now  entering, 


INERTIA  79 

more  difficulty,  perhaps,  is  to  be  anticipated  in  the 
meaning  attached  to  the  terms  employed  than  in 
the  actual  ideas  themselves.  Mass  is  often  equiva- 
lent to  "  weight,"  but  here  it  is  not  so.  The 
mass  of,  meaning  the  quantity  of,  matter,  is  a 
fundamental  idea,  while  weight  is  a  derived  idea 
due  to  the  earth's  attraction.  A  given  quantity  of 
matter  throughout  the  universe  has  an  un- 
changing mass.  Its  weight,  of  course,  depends 
upon  the  proximity  and  mass  of  the  world 
attracting  it.  What  then  is  the  measure  of 
mass  as  distinct  from  weight  ?  Weight  is,  as  a 
matter  of  fact,  invariably  used  on  the  earth  to 
measure  mass  because  it  is  so  convenient.  Yet  if 
we  can  imagine  ourselves  isolated  in  space  at  a  great 
distance  from  all  worlds  with  a  given  quantity  of 
matter  it  is  desired  to  know  the  mass  of,  we  should 
still  have  no  difficulty  in  distinguishing  the  greater 
mass,  say  of  a  sphere  of  lead,  from  the  lesser  mass 
of  a  similar-sized  sphere  of  wood.  We  should  know 
the  difference  by  the  difference  of  inertia.  If  we 
struck  each  a  similar  blow  the  wood  sphere  would 
start  to  move  many  times  as  fast  as  the  lead  sphere. 
Neither  would  have  appreciable  weight  under  these 
circumstances,  but  their  relative  inertia  would  still 
be  in  proportion  to  their  masses.  A  collision 
between  two  "weightless"  railway  trains  meeting  in 
mid-space  would  work  just  as  much  havoc  to  the  trains 


8o  CATHODE-RAY  AND  $-P ARTICLES 

as  it  would  if  it  occurred  at  the  same  speed  upon  the 
earth.  Hence,  when  a  physicist  speaks  of  the  "mass" 
of  a  /3-ray  particle,  or  of  a  cathode-ray  particle,  no 
considerations  of  weight  are  in  his  mind. 

Sir  J.  J.  Thomson,  first  with  these  cathode-rays, 
afterwards  with  the  /3-rays,  showed  how  it  was 
possible  to  determine  the  velocity,  the  charge,  and 
the  mass  of  the  particles  which  constitute  them. 

The  application  of  these  methods  resulted  in 
the  proof  that  the  charge  and  the  mass  of 
the  /3-particle  were  identical  with  that  of  the 
cathode-ray  particle  of  vacuum  tubes,  but  the 
velocity  of  the  /3-particle  was  far  higher  than  that 
of  the  fastest  known  cathode-ray.  Thus  the 
/3-particle  ejected  from  the  radium  atom  was 
already  known.  It  is  true  it  is  ejected  more 
violently  by  radium  than  in  any  previously  known 
case,  but  in  its  essential  characteristics,  its  charge, 
or  the  quantity  of  electricity  it  carries,  and  its  mass 
— it  is  the  same  particle  as  Sir  William  Crookes 
dealt  with  in  his  vacuum  tubes  thirty  years  ago. 
He  christened  them  in  a  prophetic  moment  with  the 
name  of  "  Radiant  Matter,"  and  was,  like  many 
another  prophet,  ridiculed  for  his  pains. 

The  cathode-ray  particle,  and  also  the  /3-ray 
particle,  were  found  to  carry  the  same  amount  of 
electricity  as  the  charged  hydrogen  atom.  Hence, 


CATHODE-RAY  AND  ^-PARTICLES  81 

whatever  else  the  /3-particle  of  radium  is,  it  is  cer- 
tainly an  atom  of  negative  electricity.  With  regard  to 
the  velocity,  just  as  the  mass  of  these  particles  is 
smaller  than  any  known  material  particle,  their  ve- 
locity is  appropriately  almost  inconceivably  greater 
than  that  of  any  previously  known  material  particle. 
It  approaches  that  of  light  itself,  which  has  a  velocity 
of  185,000  miles  per  second.  The  average  velocity 
of  the  cathode-ray  particle  of  the  vacuum  tube  is 
from  5,000  to  10,000  miles  per  second  ;  while  that 
of  the  fastest  of  the  /2-particles  of  radium  is  so  nearly 
that  of  light  as  to  be  indistinguishable  from  it. 
Most  of  the  /3-rays,  however,  travel  with  a  velocity 
from  40  to  80  per  cent,  of  that  of  light. 

This  is  one  of  the  most  general,  as  it  is  one 
of  the  most  remarkable,  features  about  radium. 
The  effects  produced  by  its  rays,  even  the  rays 
themselves  in  some  part,  are  not  entirely  new. 
They  can  be  simulated  to  some  extent  by  arti- 
ficial means.  In  passing  from  the  effects  produced 
artificially  to  those  produced  by  radium  spontane- 
ously, we  are  aware  of  great  resemblances,  and  at 
the  same  time  of  great  differences.  By  the  use  of 
exceedingly  powerful  electrical  appliances,  and  the 
expenditure  of  a  considerable  amount  of  energy, 
we  can  simulate  to  some  extent  the  /3-rays  of 
radium,  but  no  instrument  maker  at  the  present 
time  can  provide  you  with  the  means  of  impressing 
G 


THE  "RADIUM  CLOCK" 


upon  the  artificially  generated  cathode-ray  electron 
of  the  Crookes'  tube  more  than  a  small  fraction 
of  the  velocity  with  which  the  /3-ray  electron  is 
being  spontaneously  expelled  from  radium.  It  is 
the  same  in  other  matters.  The  utmost  we  are 
able  to  effect  by  the  most  power- 
ful forces  at  our  disposal  falls  far 
short  of  what  is  being  done  by 
a  mere  speck  of  matter  under- 
going atomic  disintegration. 

Before  leaving  the  subject  of 
/3-rays,  I  have  to  show  you  an 
interesting  instrument  devised  by 
Professor  Strutt  and  popularly 
called  the  radium  clock  (Fig.  15). 
It  is  the  nearest  approach  to  per- 
petual motion  that  has  yet  been 
devised,  and  it  consists  of  a  gold- 
leaf  electroscope,  worked  by  the 
negative  electricity  carried  away 
from  the  radium  by  means  of  the 
/3-rays.  A  few  milligrams  of  a 
salt  of  radium  are  contained  in  a 
thin-walled  closed  glass  tube,  A,  through  which  the 
/3-rays  can  easily  penetrate,  and  this  tube  is  sup- 
ported from  an  insulating  rod  of  quartz,  B,  within 
a  highly  exhausted  glass  vessel.  The  tube  in  turn 
carries  at  its  lower  end  two  gold  leaves,  C,  after  the 


FIG.  15. 


THE  "RADIUM  CLOCK"  83 

manner  of  an  electroscope.  The  £-rays  shot  out  from 
the  radium  carry  away  negative  electricity,  and  there- 
fore the  radium  itself  left  behind  becomes  positively 
charged.  The  gradual  accumulation  of  this  charge 
causes  the  gold  leaves  attached  to  the  tube  to 
diverge  little  by  little,  until  they  touch  the  sides  of 
the  vessel  and  are  discharged,  when  the  cycle  of 
operations  recommences.  The  instrument  on  the 
table  (Fig.  16,  opposite  p.  66)  was  constructed  many 
years  ago,  and  has  been  functionating  about  once 
every  three  minutes  ever  since.  There  is  no  reason 
why  it  should  not  do  so  for  at  least  a  thousand  years 
more,  though  at  a  slowly  decreasing  rate. 

The  methods  we  have  been  considering  which 
led  to  the  elucidation  of  the  real  nature  of  the 
/3-rays — the  determination  of  the  nature  of  the 
expelled  particle,  its  mass,  charge,  and  velocity — 
have  been  applied  successfully  also  to  the  elucida- 
tion of  the  real  nature  of  the  a-rays,  though  here 
the  task  was  almost  infinitely  more  difficult  experi- 
mentally. Rutherford,  to  whom  we  owe  our  know- 
ledge of  this  subject,  worked  for  a  long  time  before 
he  could  detect  any  influence  produced  by  the  most 
powerful  magnets  on  the  course  of  the  a-rays,  so 
slight  and  insignificant  it  is  compared  with  the 
effect  on  the  /3-rays.  Finally,  he  proved  that  the 
a-rays  were  deflected  both  by  electric  and  by  mag- 
netic forces,  but  to  an  extent  of  the  order  of  one 


84  CHARGE    OF  THE   ^.-PARTICLE 

thousandth  part  of  the  effect  that  the  /3-rays  suffer 
under  similar  circumstances.  The  deviation  of  the 
a-rays,  moreover,  is  in  the  opposite  direction  to 
that  of  the  /3-rays.  Where  the  /3-rays  are  coiled 
clock -wise,  for  example,  the  a-rays  would  tend  to 
turn  counter  clock-wise.  By  these,  and  numerous 
other  experiments,  it  has  been  shown  that  the 
a-rays  consist  of  positively  charged  particles.  The 
a-particle  is  however  not,  like  the  /3-particle, 
only  a  disembodied  electrical  charge.  It  is  a 
charged  material  atom.  At  first  it  was  thought  to 
be  twice  as  heavy  as  the  hydrogen  atom,  on  the 
assumption  that  it  was  charged  with  a  single 
"atom"  of  positive  electricity.  Now,  however,  it 
has  been  proved  to  carry  two  charges  of  positive 
electricity,  and  to  be  an  atom  four  times  as  heavy 
as  hydrogen.  This  is  in  accord  with  the  whole  of 
the  rest  of  the  evidence  of  radioactive  changes  still 
to  be  considered,  which  points  unmistakably,  though 
indirectly,  to  the  conclusion  that  the  a-particle  is  an 
atom  of  the  element  helium.  The  atomic  weight 
of  helium  is  four,  or,  in  other  words,  the  helium  atom 
is  four  times  as  massive  as  the  hydrogen  atom, 
which  is  always  taken  as  unity. 

Waiving  the  case  of  the  /3-rays  which,  as  we 
have  seen,  are  electrical  rather  than  material  in 
nature,  the  a-rays  of  the  radioactive  substances 


VELOCITY  OF  ^.-PARTICLE  85 

furnish  without  doubt  one  of  the  most  wonderful 
phenomena  at  present  known.  If  radium  did 
nothing  else  but  send  out  these  a-particles,  that 
alone  would  of  itself  constitute  a  new  epoch  in 
our  knowledge  of  nature.  Take  their  velocity,  for 
instance,  which,  though  lower  on  the  average 
than  that  of  the  /3-rays,  reaches  in  some  cases 
the  very  handsome  value  of  over  12,000  miles 
per  second.  This  is  hundreds  of  times  faster  than 
the  next  fastest  known  material  thing  moving  in 
earth  or  air  or  space.  The  swiftest  flight  known 
previously  is  that  of  some  of  the  shooting  stars, 
which  attain  sometimes  to  a  speed  of  from  twenty 
to  forty  miles  a  second,  and  from  the  attack  of 
which  we  are  largely  protected  by  the  fact  that 
their  velocity  is  so  great  that  they  are  quickly 
dissipated  into  vapour  by  the  simple  resistance  of 
the  air.  While  such  a  meteor  was  traversing  the 
distance  to  the  moon  an  a-particle  would,  given  an 
unimpeded  path,  reach  the  sun. 

Such  a  velocity  multiplied  by  itself,  or  squared, 
gives  us  a  measure  of  the  energy  possessed  by  the 
a-particles.  If  their  velocity  is,  say,  half  a  thousand 
times  faster  than  any  previously  known,  the  kinetic 
energy  they  possess  is,  mass  for  mass,  a  quarter  of 
a  million  times  greater  than  any  we  have  ever  had 
to  do  with  before.  In  this  fact  lies  the  key  to 
many  of  the  surprising  revelations  of  radium. 


86  ENERGY  OF  a.-P ARTICLE 

When  we  speak  of  being  able  to  detect  the  effect 
of  a  single  a-particle,  and  therefore  of  a  single 
atom  of  matter,  we  mean  the  detection  of  its 
energy,  which  is  a  quarter  of  a  million  times  as 
great  as  that  of  any  other  kind  of  atom  known  to 
us.  Similarly,  when  we  speak  of  being  able  to 
detect  in  a  few  seconds  by  radioactive  methods 
the  course  of  a  change  which  would  have  to  pro- 
ceed continuously  for  geological  epochs  before  it 
produced  an  effect  detectable  by  the  most  sensitive 
chemical  test,  it  is  because,  firstly,  we  detect  the 
energy  evolved  by  the  change,  not  the  change 
itself;  and,  secondly,  because  the  energy  is  at  once 
so  relatively  enormous  and  at  the  same  time  so 
much  more  easily  detected  compared  with  any 
other  kind  of  energy  outburst  previously  known 
to  us. 

Matter  moving  with  the  speed  of  10,000  miles 
a  second  is  so  novel  and  strange  to  us  at  present 
that  it  is  doubtful  whether  our  ordinary  concep- 
tions afford  much  guide  or  analogy.  The  muzzle- 
velocity  of  a  cannon-ball,  for  instance,  is  a  small 
fraction  of  one  mile  per  second.  Now  we  have 
seen  that  the  a-particle  of  radium  is  capable  of 
traversing  very  thin  aluminium  leaves  and  also 
several  inches  of  gaseous  air.  It  is  extremely 
interesting  to  inquire  what  happens  during  the 


A    QUOTATION  FROM  PROFESSOR  BRAGG    87 

collision  of  an  a-particle  with  a  molecule  of  gas  or 
metal.     Some  at  least  of  these  collisions  must  be 
full   and    direct,    not    simple   grazing   or   glancing 
coincidence  ;  and  it  seems  at  first  sight  difficult  to 
believe   that  an  a-particle  striking  a  gas-molecule 
full  and  fair  should  not  be  stopped,  however  fast  it 
is  moving.      Nevertheless,  it  is  not  so.      Upon  this 
matter  the  researches  of  Bragg  and  his  colleagues 
have    thrown    a    flood    of   light.      His    conclusions 
are   as    remarkable  as   they   are    definite.      "  Each 
a-particle  pursues  a  rectilinear  course,  no   matter 
what  it  encounters  ;  it  passes  through  all  the  atoms 
it  meets,  whether  they  form  part  of  a  solid  or  a 
gas  (or,  in  all  probability,  of  a  liquid),  suffering  no 
deflection    on    account   of   any  encounter   until,   at 
any  rate,  very  near  the  end  of  its  course.  ...  A 
thin  metal  plate  may  be  placed  in  the  way  of  the 
stream,  and  so  rob  every  particle  of  some  of  its 
energy,  but  not  a  single  one  is  brought  to  rest  by 
collision    with    the   atoms    of   the  metal,   and   the 
number    of    particles    in    the   stream    remains   un- 
changed."    Surely  this  vivid  picture  of  the  flight 
of    a    swarm    of    a-particles    raises    anew    the    old 
metaphysical  conundrum  of  the  schoolmen,  whether 
two  portions  of  matter  could  occupy  the  same  space 
at  the  same  time.     For  the  only  possible  meaning 
of  Professor  Bragg's  conclusion  is  that  the  a-particle 
must  go  clean  through  the  atoms  of  matter  it  pene- 


88  INTERPENETRAT1ON  OF  ATOMS 

trates  as  though  they  were  not  there,  and  therefore 
at  the  instant  of  collision  the  two  atoms  do  occupy 
the  same  space  at  the  same  time.  This  power  of 
the  interpenetration  of  masses  may  be  one  of  the 
peculiar  properties  of  matter  moving  at  these,  what 
may  be  termed  ultra-material,  velocities.  We  know 
for  certain  it  is  not  a  normal  property  of  matter. 
The  only  apparent  consequence  of  the  passage  of 
the  a-particle  through  the  atoms  it  encounters  is 
that  it  ionises  them,  that  is,  they  become  charged, 
some  with  +  and  some  with  -  electricity,  after  the 
collision.  There  is  little  doubt  that  the  +  charge 
or  charges  on  the  a-particle  itself  can  be  explained 
in  the  same  way.  Whereas  in  the  case  of  the 
/3-particle  the  charge  of  electricity  is  the  particle, 
in  the  case  of  the  a-particle  the  charge  is  in  all 
probability  a  secondary  consequence  of  the  velocity 
with  which  the  particle  is  moving.  At  least  it  is 
certain  that  no  atom  moving  at  10,000  miles  a 
second  would  continue  uncharged.  The  very  first 
collision  with  an  atom  of  matter  would  "knock  out 
an  electron,"  that  is,  charge  the  moving  particle 
positively. 

Since  the  above  quotation  was  written  it  has 
been  proved  that  some  of  the  a-particles  do  suffer  a 
deflection  or  scattering  in  their  passage  through 
matter.  For  the  vast  majority  of  the  a-particles 
this  deflection  is  exceedingly  slight,  but  for  a  very 


SCATTERING   OF  a-PARTJCLES  89 

small  proportion  of  the  whole  the  deflection  may  be 
so  great  as  practically  to  turn  the  a-particle  back 
the  way  it  came.  This  is  extremely  interesting. 
Since  the  ^-particles  alone  have  access  to  the  real 
interior  of  the  atom  of  matter,  it  is  quite  possible 
that  a  close  study  of  this  phenomenon  may  result 
in  information  being  obtained  as  to  what  the  atoms 
of  matter  consist  of.  Hitherto  science  has  been 
completely  confined  to  the  external  characteristics 
of  atoms,  and  it  is  hoped  that  the  a-particles,  after 
their  passage  through  these  atoms,  will  afford  some 
clue  as  to  the  nature  of  the  unknown  territory  which 
they  have  traversed. 

The  quotation  from  Professor  Bragg  (p.  87)  pur- 
sued the  question  of  what  happens  to  the  a-particle 
on  collision  only  as  far  as  the  initial  stages.  Each 
atom  of  matter  penetrated  robs  the  a-particle  of 
some  of  its  energy,  and  its  velocity  is  therefore 
diminished  as  it  pursues  its  path.  But  the  more 
slowly  it  moves  the  more  energy  is  withdrawn  from 
it  in  passing  through  any  given  obstacle.  In  addi- 
tion, the  slower  it  moves  the  more  easily  is  it 
deviated  from  its  course,  or  scattered.  In  conse- 
quence, the  speed  is  more  and  more  quickly  reduced 
as  the  end  of  its  path  is  approached,  and  the  a-par- 
ticle thus  passes  out  of  the  range  of  detection  some- 
what suddenly. 

By  an  ingenious  arrangement,  C.   T.  R.  Wilson 


90  MAKING   2" HE  PATHS 

has  succeeded  recently  in  making  the  paths  of  many 
of  the  new  radiations  in  air,  or  other  gas,  visible 
to  the  eye,  and  in  actually  photographing  them. 
These  rays  ionise  the  gas,  and  leave  in  their  tracks 
columns  of  ions,  which  are  molecules  of  the  gas 
carrying  an  electric  charge,  and  which,  although 
really  moving  about  like  all  gaseous  molecules  at 
great  speed,  are,  by  comparison  with  the  much 
swifter  rays  producing  them,  almost  at  rest.  Now 
these  ions,  the  negative  variety  more  easily  than 
the  positive  ions,  afford  nuclei  for  the  condensation 
of  moisture  from  a  supersaturated  atmosphere. 
Dust  plays  the  same  part,  but  all  dust  can  readily 
be  removed.  When  moist  air  in  a  closed  space  is 
suddenly  expanded  the  air  is  cooled  and  the  mois- 
ture condenses  as  mist  or  rain  on  the  dust  particles 
and  carries  them  down,  so  freeing  the  air  from  such 
impurities.  If  the  pure  air  is  now  suddenly  ex- 
panded within  certain  well-defined  limits,  in  the 
absence  of  ions  or  dust,  no  condensation  is  produced. 
But  if  the  air  is  traversed  by  any  of  the  new  ionis- 
ing radiations,  the  tracks  of  the  rays,  when  the 
ionisation  chamber  is  suitably  illuminated,  appear 
momentarily  as  long  spider-threads  of  mist  when- 
ever the  air  is  suddenly  expanded  and  chilled.  If  a 
flash  of  light  is  arranged  to  take  place  just  after  the 
expansion,  the  threads  may  be  photographed.  The 
tracks  left  by  the  ^-particles  are  almost  all  perfectly 


OF  RAYS    VISIBLE  91 

straight,  but  a  very  few  show  abrupt  large  deflec- 
tions, and  sometimes  actually  the  direction  of  travel 
is  nearly  reversed.  The  /3-rays,  on  the  other  hand, 
give  very  zigzag  tracks.  These  rays  are  known 
to  be  scattered  and  turned  very  readily  by  their 
encounter  with  the  molecules  of  matter.  The  ex- 
periments have  also  thrown  light  on  the  nature  of 
the  y-rays,  and  have  made  it  appear  probable  that 
these  rays  do  not  ionise  the  gas  directly,  but  first 
cause  the  molecules  struck  to  emit  a  kind  of  cathode- 
or  ^-radiation,  and  it  is  these  secondary  radiations 
which  produce  the  ionisation.  Fluorescent,  photo- 
graphical,  and  electrical  actions  all  cease  simul- 
taneously. It  is  estimated  that  at  the  moment  the 
a-particle  ceases  to  be  detectable  it  is  still  moving 
with  the  velocity  of  several  thousand  miles  a  second. 
For  all  that  is  known  the  particle  may  then  suffer  a 
sudden  stop,  or  it  may  continue  its  course  without 
ionising  the  atoms  it  encounters. 

For  us,  who  are  concerned  for  the  most  part  with 
the  broad  limitations  of  our  past  and  present  know- 
ledge, the  most  interesting  feature  of  this  pheno- 
menon is  that  it  indicates  quite  definitely  that  an 
a-particle  expelled  with  an  initial  velocity  below 
several  thousand  miles  a  second  could  not  by  any  of 
the  present  known  methods  be  detected.  Any  of 
the  apparently  stable  and  non-radioactive  elements 
might  be  disintegrating  and  expelling  a-particles, 


92  UNDISCOVERABLE  PROCESSES 

but  if  these  did  not  attain  this  limiting  speed  we 
should  have  no  evidence  of  the  fact.  It  is  really  by 
a  somewhat  slender  margin  of  velocity  that  the 
a-particles  have  come  within  our  knowledge  at  all. 
The  light  we  have  gained  has  but  served  to  intensify 
the  darkness  by  which  we  are  surrounded  on  all 
sides.  Processes  similar  to  and  but  little  less 
energetic  than  those  which  produce  radioactivity, 
may  be  going  on  unsuspected  everywhere  around 
us,  without  producing  any  yet  detectable  effects. 
Radioactivity  is  to  be  regarded  rather  as  a  bene- 
volent hint  given  to  us  by  Nature  into  secrets 
we  might  never  have  guessed,  rather  than  as  the 
necessary  and  invariable  concomitant  of  the  pro- 
cesses of  atomic  disintegration. 


CHAPTER  V. 

From  where  does  the  energy  of  radium  come  ? — The  two  alternatives 
and  their  consequences — The  internal  energy  of  matter — Atomic 
disintegration — Disintegration  in  cascade — The  successive  out- 
bursts of  energy  from  otherwise  impalpable  quantities  of  matter — 
The  emanation  of  radium — Its  properties — Experiments  with  the 
emanation — Its  condensation  by  liquid  air — The  infinitesimal 
quantity  of  the  emanation  from  radium — The  chemical  nature  and 
atomic  weight  of  the  emanation — The  energy  evolved  by  the 
emanation — The  decay  of  the  emanation  and  its  reproduction  by 
radium — The  facts,  not  the  theories,  of  radioactivity  are  revolu- 
tionary— Unalterability  of  radioactive  changes — Evanescent  pro- 
ducts of  radioactive  change — All  products  equally  knowable 
whether  short-lived  or  long. 

IF  we  are  to  continue  to  regard  energy  in  the 
modern  way  as  something  having  a  definite 
existence,  we  have  to  answer  the  question,  "  From 
where  does  the  energy  of  radium  come  ?  "  That  it 
comes  from  nowhere,  or  that  it  is  being  newly 
created  out  of  nothing  by  radium,  is  a  view  it  is 
not  possible  to  entertain  for  a  moment  without 
destroying  the  basis  upon  which  nineteenth-century 
physical  science  has  largely  been  reared.  "  How 
has  it  got  the  energy  in  it  to  do  it  ? "  is  the  first 
question  that  naturally  arises  in  the  mind  with  re- 
gard to  radium,  but  obviously  we  should  first  ask, 
"  Has  it  the  energy  in  it  ? " 

93 


94      SOURCE    OF  THE   ENERGY  OF  RADIUM 

If  the  doctrine  of  energy  is  true,  there  are  for- 
tunately only  two  possible  alternatives  to  be  con- 
sidered. Either  the  energy  must  be  derived  from 
within  the  radium,  which  we  shall  call  the  first,  and 
as  we  think  the  true,  alternative,  or  it  must  be 
supplied  from  outside  the  radium,  and  this  we  shall 
call  the  second  alternative.  This  simple  narrowing 
down  of  all  the  possible  issues  to  two  alternatives  may 
appear  to  you  somewhat  trite,  but  in  reality  it  carries 
with  it  far  more  than  appears  on  the  surface.  In 
the  first  place,  being  an  intrinsic  property  of  the 
element,  radioactivity  is  therefore  a  property  of  the 
atom,  and  if  we  take  the  first  alternative  and  say 
the  energy  comes  from  within,  it  means  from  within 
the  atom,  and  therefore  that  there  must  exist  an 
enormous  and  not  previously  suspected  store  of 
energy  in  matter,  or  at  least  in  radioactive  matter, 
in  some  way  associated  with  its  atoms  or  smallest 
integral  parts. 

On  the  second  alternative,  which  has  often 
been  advanced,  radium  acts  merely  as  a  transform- 
ing mechanism.  There  are  electrical  transformers 
dotted  all  over  this  city,  receiving  the  economically 
transmitted  but  dangerous  high-tension  currents 
from  the  central  power  station  and  delivering  the 
comparatively  safe  low-tension  currents  to  your 
houses,  which  are  wasteful  to  transmit  for  long 
distances.  Are  the  atoms  of  radium  acting  as  the 


THE    TWO  ALTERNATIVES  95 

transformers  of  a  mysterious  and  hitherto  unknown 
source  of  external  energy,  first  receiving  it  and  then 
delivering  it  up  again  in  a  form  which  can  be  recog- 
nised ?  It  may  be  said  at  once  that  so  vague  a 
view,  postulating  the  existence  of  illimitable  and 
mysterious  supplies  of  energy  from  without,  cannot 
be  directly  disproved.  At  first  it  seemed  to  provide 
a  way  of  escape  from  some  of  the  more  unpalatable 
logical  consequences  of  the  first  alternative  and  was 
eagerly  adopted.  In  reality,  instead  of  a  way  of 
escape,  it  proves  to  be  a  veritable  will-of-the-wisp, 
luring  on  its  followers  beyond  the  limits  of  credulity 
into  a  quagmire  of  unsubstantial  hypotheses,  so 
bottomless  and  unreal  that  even  the  facts  of  radium 
are  a  wholly  inadequate  justification,  and,  even  so, 
incapable  of  throwing  any  light  on  the  facts  when 
these  are  more  nearly  examined.  Nevertheless,  we 
must  pursue  both  alternatives  impartially,  if  only 
to  leave  no  doubt  that  both  have  only  to  be  fairly 
considered  for  one  to  be  dismissed. 

On  the  second  alternative  the  radium  owes  its 
activity  to  a  supply  of  energy  from  outside.  One 
has  only  to  isolate  the  transformers  which  light  this 
city  from  all  connection  with  the  outside  central 
station  to  plunge  the  city  in  darkness.  But  we 
have  seen  that  to  quench  radioactivity  or  to  modify 
it  in  any  way  is  one  of  the  things  science  cannot 
do.  Experiment  has  proved  that  even  in  the 


96      SOURCE    OF  THE  ENERGY  OF  RADIUM 

natural  state  in  the  mine,  hundreds  of  feet  deep 
down  in  the  earth,  pitchblende  exhibits  its  normal 
radioactivity.  So  that  if  it  derives  its  energy  from 
without,  this  must  be  of  a  kind  entirely  different 
from  any  at  present  known,  for  it  must  be  capable 
of  traversing  without  loss  hundreds  of  feet  of  solid 
rock.  This  is  as  far  as  we  need  pursue  the  second 
alternative  for  the  moment.  Provided  we  can  call 
into  existence  a  new  kind  of  radiant  energy  un- 
limited in  amount,  permeating  all  space  and  unim- 
peded by  passage  through  matter  of  any  thickness, 
we  may,  but  only  so  far  as  we  have  yet  gone,  seek 
a  bare  explanation  of  the  energy  of  radium  on  the 
second  alternative.  Such  a  view  would  accord  at 
first  sight  with  the  continuous  and  permanent 
activity  of  radium  for  an  indefinite  time,  and  there 
would  be  no  reason  why  radioactivity,  however 
intense  and  powerful,  should  decay  or  diminish 
with  the  lapse  of  time. 

But  if  the  first  alternative  is  true,  and  the  energy 
comes  from  within,  large  as  the  store  of  energy  in 
the  atom  must  be  to  explain  radioactivity  it  cannot 
be  infinite,  and  therefore  it  is  to  be  expected  that 
the  activity  will  slowly  decay  with  the  lapse  of  time. 
If  two  radioactive  bodies,  one  much  more  power- 
fully radioactive  than  the  other,  are  compared 
together,  it  is  to  be  expected  on  this  view  that  the 
activity  of  the  more  powerful  body  will  decay  faster 


THE    TWO   ALTERNATIVES  97 

than  that  of  the  other.  But  for  both  a  time  will 
come,  as  soon  as  the  internal  stores  of  energy  are  ex- 
hausted, when  the  radioactivity  will  come  to  an  end. 

By  far  the  most  important  consequence  of  the 
first  alternative,  however,  has  still  to  be  considered. 
Radium,  if  we  call  by  that  name  the  substance  con- 
taining the  unevolved  store  of  energy,  can  no 
longer  be  radium  when  the  energy  is  lost.  Coal 
is  not  coal  after  it  is  burnt.  When  energy  is  ob- 
tained from  matter  the  matter  changes,  and  before 
it  can  be  regained  in  its  former  state  the  energy 
evolved  must  be  put  back.  In  no  case  is  it  possible 
for  matter  to  part  with  its  store  of  energy  and  re- 
main the  same,  for  otherwise  you  will  readily  see  a 
perpetual  motion  machine  would  be  easy  enough 
to  construct.  Indeed,  most  of  those  attempted 
involved  this  impossible  assumption. 

But  we  have  seen  that  if  the  energy  is  stored 
up  in  the  radium  it  must  be  within  the  atom, 
and,  therefore,  if  radium  changes,  it  must  be 
a  change  of  the  atom  and  of  the  element  itself. 
This  change  of  an  element  would  be  transmutation, 
which  is  a  more  fundamental  and  deep-seated  change 
than  chemical  change  or  any  known  kind  of  material 
change,  and  until  the  discovery  of  radioactivity  such 
changes  certainly  had  never  been  observed.  If  the 
energy  of  radium  comes  from  within,  radium  must 
be  suffering  a  spontaneous  kind  of  transmutation 


H 


98       THE  INTERNAL   ENERGY  OF  MATTER 

into  other  elements.  So  that,  if  we  would  avoid 
the  necessity  of  believing  in  the  process  of  trans- 
mutation, not  as  a  vague  possibility,  for  example, 
in  the  sun  and  stars,  under  some  unattainable  trans- 
cendental condition,  but  as  actually  going  on  im- 
perturbably  around  us,  which  the  first  alternative 
demands,  we  must  seek  a  way  of  escape  on  the 
second  alternative  which  requires  •  none  of  these 
bewildering  heresies,  but  simply  transfers  the 
mystery  from  the  radium  to  the  great  external 
unknown,  and  leaves  it  there  in  good  company 
with  many  of  a  similar  kind. 

At  this  stage  it  is  well  to  ask  the  question,  Is 
there  anything  opposed  either  to  reason  or  to  prob- 
ability in  the  view  that  the  energy  evolved  from 
radium  is  actually  derived  from  an  existing  pre- 
viously unsuspected  internal  store  within  the  atom, 
and  that  in  this  process  the  element  suffers  a  trans- 
formation into  other  elements  ?  How  is  it  that 
such  enormous  stores  of  energy  in  matter  have 
remained  so  long  unknown  ? 

One  of  the  most  elusive  features  of  energy  is 
that  you  cannot  say  by  mere  observation,  or  by  the 
use  of  any  instrument,  how  much  or  how  little  is 
stored  up  in  any  kind  of  matter.  For  example, 
this  flask  contains  a  large  quantity  of  an  oily  yellow 
liquid.  We  cannot  tell  by  simple  inspection  the 
amount  of  energy  stored  up  in  this  fluid.  It  may 


THE  INTERNAL   ENERGY  OF  MATTER      99 

be  some  quiet  and  harmless  oil,  which  can  be 
shaken  vigorously  with  impunity,  or  it  may  be 
nitro-glycerine,  one  of  the  most  dangerous  and 
powerful  explosives.  Something  more  than  obser- 
vation is  necessary  to  tell  us  the  amount  of  energy 
that  may  be  stored  within  this  substance,  possibly 
only  awaiting  a  slight  shock  to  be  evolved.  The 
only  way  to  find  out  is  to  try  to  explode  it  as 
thoroughly  as  we  can,  and  then  if  it  will  not  ex- 
plode we  may  conclude  that,  as  far  as  we  know,  it 
has  no  latent  store  of  energy  waiting  to  be  loosed 
from  prison. 

Explosion  is  merely  a  very  rapid  and  violent  type 
of  chemical  change,  and  the  same  general  idea  holds 
good  for  all  the  changes  it  is  possible  for  matter  to 
undergo.  We  may  determine  the  energy  evolved 
or  absorbed  in  any  change,  that  is,  in  the  passage 
from  one  kind  of  matter  to  another  kind.  We  have 
no  means  of  telling  the  absolute  amount  of  energy 
in  any  kind  of  matter.  But  the  one  thing  of  which 
the  chemist  is  positive  is  that  in  all  the  material 
changes  matter  undergoes — radioactivity  being  ex- 
cepted — the  elements  do  not  change  into  one 
another,  but  remain  in  their  various  compounds 
essentially  unaltered.  If  transmutation  were  pos- 
sible, and  one  element  could  be  changed  into 
another,  it  would  be  easy  to  measure  the  difference 
in  the  amount  of  energy  of  the  two  elements. 


ioo  THE  STABILITY  OF  ELEMENTS 

As  it  is,  the  internal  energy  of  the  elements 
remains  always  unaffected  by  previously  known 
material  changes,  and  therefore  till  recently  quite 
unknowable. 

Before  we  can  find  out  how  much  or  how  little 
energy  is  internally  associated  with  the  atoms  we 
must  be  able  to  study  a  case  of  transmutation. 
The  great  stability  of  all  elements  under  all  condi- 
tions— even  in  the  sun  the  identical  elements  which 
we  know  here  persist,  if  we  can  rely  on  the  evi- 
dence of  the  spectroscope — is  well  in  accord  with 
the  view  that  all  the  elements  contain  a  very  large 
store  of  internal  energy,  which  is  never  released  in 
ordinary  changes,  but  which  makes  them  indifferent 
to  changes  in  their  environment.  Thus  the  internal 
kinetic  energy  of  a  torpedo  containing  a  revolving 
gyrostat  makes  it  successfully  resist  deflection  from 
its  course  by  the  wind  and  waves.  The  internal 
energy  of  the  solar  system,  taken  as  a  whole,  is  the 
sole  reason  why  it  continues  to  exist  as  a  system 
and  does  not  drift  apart. 

So  far  then  from  there  being  anything  opposed  to 
reason  or  probability  in  the  view  that  the  atom  of 
the  element  contains  a  great  and  hitherto  unknown 
store  of  internal  energy,  we  see  that  if  it  possessed 
such  a  store  we  could  not  know  of  it  until  it 
changed,  while  the  greater  the  store  the  more  would 
it  resist  change  from  without,  and  therefore  the  less 


MINUTENESS  OF  THE  EXPECTED   CHANGES  101 

likely  should  we  be  to  suspect  its  existence.  From 
this  point  forward  we  shall  find  that  the  more  the 
apparent  objections  to  the  first  alternative  of  internal 
energy  are  faced  the  less  serious  they  appear,  while 
with  the  second  alternative  of  external  energy  the 
contrary  is  the  case. 

Having  with  these  preliminaries  somewhat  cleared 
the  ground,  I  now  wish  to  attempt  to  explain  a 
series  of  experimental  investigations  which  have 
thrown  a  flood  of  light  upon  the  nature  of  radio- 
activity. Though  by  a  superficial  or  merely  external 
observation  of  radium,  even  over  the  period  of 
a  whole  lifetime,  it  would  hardly  be  possible  to 
detect  the  least  change  of  any  kind  in  the  matter 
itself  or  any  exhaustion  of  its  output  of  energy, 
these  investigations  have  proved  that  radium,  and 
every  element  that  is  radioactive,  is  actually 
changing  in  a  very  peculiar  and  definite  way. 
These  new  changes  in  radioactivity  are  always 
excessively  minute  as  regards  the  actual  quantities 
of  matter  undergoing  change  in  any  period  of  time. 
Except  in  very  special  circumstances  they  are  quite 
beyond  the  range  of  the  most  delicate  methods  of 
investigation  previously  known  to  the  chemist. 
The  methods  employed  in  their  investigation  are 
in  the  first  place  wholly  novel,  but  they  are  none 
the  less  trustworthy  or  definite  on  that  account. 


102  THE    CASCADE   OF  CHANGES 

They  depend  on  the  important  fact  that  when  a 
radioactive  element  changes  it  does  not  as  a  rule  do 
so  once  only,  producing  in  a  single  step  the  final 
product  of  its  change.  Usually  there  are  several 
successive  changes  following  one  another,  so  to 
speak,  in  cascade.  Just  as  a  waterfall,  instead  of 
taking  one  plunge  into  a  lake,  may  cascade  in  a 
series  of  successive  leaps  from  pool  to  pool  on  the 
way  down,  so  a  radioactive  element  like  radium 
passes  in  its  change  through  a  long  series  of  inter- 
mediate bodies,  each  produced  from  the  one  pre- 
ceding and  producing  the  one  following.  Whereas, 
however,  the  first  change  is  and  must  be  slow,  the 
subsequent  changes  may  be,  and  usually  are,  rela- 
tively far  more  rapid/  But  for  the  existence  of 
these  ephemeral,  rapidly  changing,  intermediate 
substances,  continually  being  produced  and  as 
continually  changing,  it  is  safe  to  say  the  mystery 
of  radium  would  to-day  be  still  unsolved. 

Picture  to  yourselves  exactly  what  this  problem 
involves.  Out  of  a  remote,  and  so  far  as  we  know 
unlimited,  past  this  world  has  gradually  come  into 
the  state  we  find  it  to-day,  and  what  we  find  is 
that  there  is  a  process  known  as  radioactivity  still 
spontaneously  going  on  in  matter  in  its  natural 
state  as  it  is  dug  out  of  the  earth,  which  we  cannot 
in  any  way  stop  or  retard,  and  which  we  recog- 
nise as  the  intrinsic  property  of  certain  chemical 


EXISTING   METHODS  INADEQUATE        *°3 

elements.  We  must  conclude,  until  we  have 
evidence  to  the  contrary,  that  radioactivity  is  not 
a  process  which  has  started  recently,  or  that  it  is 
confined  to  the  particular  epoch  of  the  earth's 
history  we  are  now  living  in.  So  long  as  the 
radioactive  elements  have  existed  this  process 
must  have  been  going  on,  and,  if  we  are  forced 
to  the  conclusion  that  the  radioactive  elements 
are  changing,  is  it  not  obvious  that  the  changes 
must  be  excessively  slow  for  any  of  the  radio- 
active elements  to  have  survived  ?  What  could 
the  methods  of  chemistry  avail  in  such  a  search  ? 
Delicate  as  these  are  to-day,  beyond  the  limit 
of  what  was  even  conceivable  a  hundred  years 
ago,  infinitely  finer  and  more  sensitive  methods 
are  required. 

The  geologists  tell  us,  and  we  shall  find  in  radio- 
activity only  confirmation,  that  the  earth  has 
existed  in  much  the  same  physical  condition  as 
it  exists  to-day  for  hundreds  if  not  thousands  of 
millions  of  years.  A  chemist  could  probably  in 
many  cases  detect  the  change  of  one  thousandth 
part  of  one  element  into  another,  whereas  we  shall 
come  to  see  that  for  even  such  a  small  fraction  of  a 
primary  radioactive  element  to  change  a  period  of 
the  order  of  a  million  years  would  almost  certainly 
be  necessary. 

You  all  know  the  stride  that  chemistry  took  for- 


i°4     IMPONDERABLE    QUANTITIES   OF  MATTER 

wards  when  it  impressed  into  its  service  the  spectro- 
scope, and  was  able  to  detect  with  certainty  quantities 
of  new  elements  absolutely  imperceptible  in  any  other 
way.  For  example,  Bunsen  and  Kirchoff  detected 
by  the  spectroscope  the  unknown  element  ccesium 
in  the  natural  waters  of  the  Durkheim  spring  in  the 
Palatinate,  but  to  obtain  enough  caesium  for  their 
chemical  investigations  they  had  to  boil  down  forty 
tons  of  this  water.  Coming  nearer  the  present  day, 
Madame  Curie  made  an  equal  or  even  greater  step 
forward  when  she  impressed  into  the  service  of 
chemistry  the  property  of  radioactivity  and  dis- 
covered the  new  element  radium  in  pitchblende, 
though  a  ton  of  pitchblende  contains  only  two 
grains  of  radium.  But  we  must  improve  even 
on  this.  We  have  to  detect  the  change  in  a 
minute  amount  of  radium  which  is  changing  so 
slowly  that  it  appears  not  to  be  changing  at  all. 
The  actual  amount  of  new  matter  which  this  half- 
grain  of  radium  bromide  would  produce  by  its 
change  in,  say,  a  month  or  a  year,  is  a  quantity  so 
small  that  one  has  only  to  attempt  to  conceive  it  to 
be  ready  to  give  up  the  search  in  despair,  Yet  in 
a  moment  I  hope  to  show  it  to  every  one  in  this 
large  room,  and  to  demonstrate  to  you  a  few  of  its 
most  striking  properties  in  the  clearest  way. 

Were  radium  to  change  in  one  single  step  into, 
say,  lead,  which  we  believe   may   be  the  ultimate 


SUCCESSIVE   OUTBURSTS   OF  ENERGY     105 

product  in  the  main  line  of  descent,  this  would  be 
impossible.  Those  of  you  in  the  back  could  hardly 
see  a  quantity  of  lead  equal  in  quantity  to  the 
whole  of  this  radium.  How  much  less  then  could 
you  hope  to  be  shown  the  infinitesimal  fraction  of 
this  small  quantity  which  is  produced  in  a  month  or 
a  year  ?  No  chemist  has  yet  detected  lead  as  the 
final  product  of  radium,  and  our  evidence  on  this 
point  is  at  present  only  indirect,  and  not  even  very 
conclusive.  But  radium  does  not  change  all  at 
once  in  one  step.  At  least  eight  intermediate 
bodies  intervene,  each  one  of  which  is  formed 
from  the  one  preceding  it  with  an  outburst  of  energy, 
and  changes  into  the  next  with  another  outburst  of 
energy. 

A  soldier  on  a  battlefield  knows  without  any 
doubt  when  he  is  being  fired  at,  but  it  would  take 
him  a  long  and  patient  examination  to  find  out, 
and  it  would  be  a  matter  of  only  secondary 
interest,  whether  the  bullets  are  made,  say,  of 
lead  or  of  nickel.  The  energy  possessed  by  the 
flying  bullets  are  their,  to  him,  practically  im- 
portant feature.  After  the  energy  is  all  spent  the 
bullet  ceases  to  make  its  presence  felt.  So  it  is 
with  radium.  The  energy  possessed  by  the 
changing  intermediate  substances  and  evolved 
from  them  is  the  sole  but  sufficient  evidence  of 
their  existence.  After  the  energy  is  all  spent  and 


io6     ACHIEVEMENTS  OF  THE  NEWER  METHOD 

the  change  is  complete,  only  a  most  minute  and 
patient  examination,  which  has  still  to  be  made 
complete,  will  reveal  the  chemical  nature  of  the 
minute  amount  of  dead  products  formed.  But 
before  this  stage  is  reached,  in  the  long  succes- 
sion of  energy  outbursts  which  accompany  the 
change  of  one  intermediate  form  into  the  next, 
we  have  a  succession  of  most  remarkable  and 
obvious  phenomena  which  enable  us  to  detect 
the  separate  changes  and  to  discover  the  whole 
nature  and  the  periods  of  average  life  of  all  the 
intermediate  bodies,  although  these  all  exist  only 
in  absolutely  infinitesimal  quantity,  and  not  one 
of  them  is  known,  or  probably  ever  can  become 
known,  to  the  chemist  in  the  ordinary  way.  It  is 
one  of  the  most  wonderful  triumphs  in  the  whole 
history  of  physical  science  that  such  changes 
should  have  ever  been  detected.  Let  us  turn  to 
the  main  evidence  on  which  the  view  that  radium 
is  changing  was  first  based. 

If  this  specimen  of  radium  bromide  was  dis- 
solved in  water  and  the  liquid  evaporated  down 
to  dryness  in  order  to  get  back  the  solid  com- 
pound, it  would  be  found  that  as  the  result  of  this 
very  simple  operation  the  radium  had  lost  the 
greater  part  of  its  radioactivity  in  the  process. 
The  penetrating  /3-  and  y-rays  would  have  com- 


THE  EMANATION  OF  RADIUM  107 

pletely  disappeared,  and  the  non-penetrating  a-rays 
would  only  be  one  quarter  as  powerful  as  initially. 
Then  a  strange  thing  would  happen.  Left  to 
itself  the  radium  would  spontaneously  recover  its 
lost  activity,  little  by  little  from  day  to  day,  and 
at  the  end  of  a  month  it  would  be  not  appreciably 
less  active  than  it  at  first  was,  or  as  it  now  is. 

This  appears  to  be  in  direct  conflict  with  the 
statement  previously  made  that  the  radioactivity 
of  radium  cannot  be  affected  by  any  known 
process,  but  it  is  only  apparently  so.  If  we 
study  the  process  carefully  we  shall  find  that 
when  the  radium  is  dissolved  in  water  "some- 
thing" escapes  into  the  air,  and  this  "something" 
is  intensely  radioactive.  It  diffuses  about  in  the 
air,  but  remains  contained  within  a  closed  vessel, 
if  it  is  gas-tight.  In  short,  this  "something"  is  a 
new  gas  possessing  the  property  of  radioactivity 
to  a  very  intense  degree. 

We  owe  the  greater  part  of  our  knowledge  of 
this  new  radioactive  gas  to  Professor  Rutherford, 
who  has  given  to  it  a  special  name.  He  called  it 
the  emanation  of  radium,  or,  for  short,  simply  the 
emanation.  The  vague  term  "emanation"  is, 
with  our  present  exact  knowledge  of  its  real 
nature,  apt  to  mislead.  Some,  unfortunately, 
have  used  the  term  "  emanation "  or  "  ema- 
nations "  in  speaking  of  the  various  radiations 


io8  THE  EMANATION  OF  RADIUM 

which  radium  emits,  and  which  we  have  already 
considered  in  some  detail.  Sir  William  Ramsay 
has  proposed  the  name  "Niton"  for  this  new  gas, 
in  order  to  emphasise  its  relationship  to  the  other 
argon  gases.  However,  as  similar  new  gases  or 
emanations  are  given  by  two  other  of  the  radio- 
active elements,  thorium  and  actinium,  the  original 
term  has  been  generally  retained.  The  term 
"  emanation,"  qualified  when  necessary  by  the  name 
of  the  radioactive  element  producing  it,  denotes  one 
of  these  new  gaseous  bodies,  and  it  is  necessary  not 
to  confuse  this  particular  use  with  its  older  and 
more  general  uses. 

In  the  laboratory,  half  a  mile  from  this  lecture- 
room,  I  have  a  further  quantity  of  about  half  a 
grain  of  pure  radium  bromide  which  has  been 
dissolved  in  water.  The  solution  is  kept  in  a 
closed  vessel.  This  morning  I  extracted  the 
emanation  from  the  vessel,  and  I  have  brought 
it  here  to  show  you.  The  radium  from  which  it 
was  derived  is  not  in  the  room,  it  is  still  in  the 
laboratory  half  a  mile  away.  The  emanation  is 
contained,  mixed  with  air,  in  a  little  glass  tube 
(Fig.  17)  provided  with  taps  for  its  admission 
and  extraction,  and  inside  this  tube  are  some  frag- 
ments of  the  mineral  willemite,  a  silicate  of  zinc. 
This  mineral  has  the  appearance  of  an  ordinary 
cold  greenish-grey  stone,  quite  undistinguished 
and  not  very  different  from  many  of  the  common 


FIG.  17.     Tube  containing  Willemite  used  to  exhibit  the  Radium  Emanation. 


FK;.  1 8.     The  same  tube  photographed  in  the  dark  by  its  own  phosphorescent  light. 


To  face  p.  108. 


THE  EMANATION  OF  RADIUM  109 

pebbles  of  the  road  or  sea- shore.  It  however 
possesses  the  power  of  fluorescing,  under  the 
action  of  X-rays  and  the  rays  from  radium,  with 
a  brilliant  greenish  light,  as  you  may  see  when  I 
bring  my  capsule  containing  half  a  grain  of  solid 
radium  bromide  near  to  a  block  of  the  mineral 
in  the  dark.  Let  us  now  in  the  dark  examine 
the  tube  containing  the  emanation  and  willemite 
together.  We  find  the  willemite  glowing  with 
a  most  remarkable  light.  Even  in  ordinary  lamp- 
light or  weak  daylight  the  glow  of  the  willemite 
is  clearly  visible.  Fig.  18  shows  the  tube 
(Fig.  17),  which  has  been  placed  in  front  of 
the  camera  in  the  dark  room,  and,  as  you  can 
see,  the  pieces  of  glowing  willemite  have  photo- 
graphed themselves  by  their  own  light.  In  the 
negative  the  walls  of  the  glass  tube,  which  also 
are  rendered  feebly  fluorescent  by  the  emanation, 
are  faintly  visible.  The  photograph  proved  some- 
what difficult  to  obtain,  as  the  light,  consisting 
almost  wholly  of  green  and  yellow,  is  almost  non- 
actinic  to  the  photographic  plate.  An  isochromatic 
plate  must  be  employed  and  a  long  exposure  given. 
Under  these  circumstances  the  6-  and  y-rays  from 
the  tube,  as  they  are  not  refracted  by  the  lens, 
themselves  fog  the  plate  uniformly  to  a  consider- 
able extent.  The  photograph  gives  no  idea  of 
the  beauty  of  the  original  tube.  Willemite 
glowing  in  the  emanation  of  radium  is  one  of 


no  THE  EMANATION  OF  RADIUM 

the  most  beautiful  sights  I  know,  and  considered 
with  reference  to  the  origin  of  its  light  and  all 
that  the  phenomenon  foreshadows  for  humanity, 
it  raises  feelings  which  only  a  poet  adequately 
could  express. 

What  is  the  emanation  of  radium  ?  I  shall  treat 
this  question  to-night  solely  as  though  the  emana- 
tion was  a  body  with  no  connection  whatever  with 
radium,  because  a  knowledge  of  its  own  nature  is 
necessary  before  its  real  relation  to  radium  can  be 
appreciated.  In  the  first  place,  it  is  intensely  radio- 
active on  its  own  account — that  is  to  say,  it  gives 
out  the  new  kinds  of  rays  very  similar  in  character  to 
those  given  by  other  radioactive  bodies  and  capable 
of  producing  the  same  effects.  What  I  am  about  to 
say  refers  only  to  a  tube  in  which  the  radium  ema- 
nation has  been  confined  for  some  hours.  At  first 
the  emanation  gives  only  a-  but  no  /3-  or  y-rays,  as 
we  shall  consider  more  nearly  later  (Chapter  VIII.). 

This  tube,  in  which  the  emanation  is  confined, 
glows  in  the  dark  because  the  phosphorescent 
willemite  it  contains  is  being  bombarded  by  the 
rays  from  the  emanation.  Some  of  these  rays 
penetrate  the  glass  walls  of  the  tube,  as  you  may 
see  if  I  bring  the  X-ray  screen  between  your  eyes 
and  the  tube.  Moreover,  if  a  very  thin  plate  of  metal 
is  interposed  at  the  back  of  the  screen  it  does  not 
perceptibly  diminish  the  effect,  for  the  rays  from 
a  tube  containing  the  emanation,  like  the  radium- 


THE  EMANATION  OF  RADIUM  in 

rays  themselves,  are  capable  of  penetrating  a  con- 
siderable thickness  of  metal.  They  consist,  in  fact, 
of  a-,  /3-  and  y-rays  together.  Any  of  the  other 
phosphorescent  bodies — for  example,  zinc  sulphide 
— would,  if  placed  inside  this  vessel  with  the  ema- 
nation, glow  in  its  characteristic  way  just  as  if 
exposed  to  radium  itself.  Similarly,  a  photographic 
plate  would  be  fogged  almost  instantly,  and  an  elec- 
trified silk  tassel  would  be  discharged  at  once  by  the 
rays  proceeding  from  the  emanation  confined  in 
this  tube.  The  similarity  between  the  a-rays  from 
the  emanation  and  those  from  radium  have  been 
proved  by  exact  physical  experiments. 

The  next  point  is  that  the  emanation  is  not  a 
solid  form  of  matter  dispersed  like  fine  particles 
of  smoke  in  the  air  which  carries  it.  It  is  a  true 
gas.  This  has  been  proved  by  innumerable  ex- 
periments ;  but  I  wish  to  show  you  one  which  is 
particularly  beautiful,  and  which  has,  I  think,  con- 
vinced every  one  who  has  ever  seen  it  performed 
that  the  emanation  of  radium  is  a  true  gas  with 
the  property  of  radioactivity.  It  was  first  per- 
formed by  Professor  Rutherford  and  myself  in 
Montreal  in  November,  1902.  If  the  emanation 
is  a  gas  there  ought  to  be  some  temperature, 
though,  perhaps  a  very  low  one,  at  which  it  loses 
its  gaseous  form  and  is  condensed  or  frozen.  All 
our  attempts  to  effect  such  a  condensation  at  tern- 


H2      THE   CONDENSATION  OF  THE  EMANATION 

peratures  down  to  -  100°  Centigrade  had  proved 
futile,  and  we  had  no  means  of  obtaining  the  very 
low  temperatures  now  daily  employed  in  a  modern 
laboratory.  But  a  liquid  air  machine  was  given 
to  the  laboratory  by  its  generous  founder,  and  on 
its  first  run  the  emanation  of  radium  was  success- 


FIG.   19. 


fully  condensed.  Exact  experiments  showed  that 
the  emanation  is  condensed  quite  sharply  when 
the  temperature  falls  below  —  1 50°  Centigrade  (or 
—  238°  Fahrenheit),  and  it  volatilises  and  again 
resumes  its  gaseous  state  quite  sharply  when  the 
temperature  rises  above  this.  We  shall  perform 
the  experiment  in  the  following  manner  (Fig.  19). 
To  one  of  the  tubes  of  the  vessel  containing  the 


THE  CONDENSATION  OF  THE  EMANATION     113 

emanation  is  attached  a  rubber  blowing-ball,  for 
blowing  out  the  emanation.  The  other  tube  is 
connected  with  a  U-tube  of  glass  containing  some 
fragments  of  willemite,  immersed  in  a  vessel  of 
liquid  air  and  so  kept  at  the  very  low  temperature 
of  about  -183°  Centigrade  or  -300°  Fahrenheit, 
into  which  the  emanation  is  blown.  Exposed  to 
this  extreme  cold  the  emanation  instantly  loses  its 
gaseous  state  and  condenses  in  the  tube.  To  make 
the  experiment  more  striking,  between  the  tube 
containing  the  emanation  and  the  cooled  U-tube  I 
have  interposed  several  yards  of  narrow  tubing 
which  the  emanation  has  to  traverse  before  reach- 
ing the  tube  in  which  it  condenses.  As  you  see, 
when  I  open  the  taps  and  gently  blow  a  blast  of 
air  to  sweep  out  the  emanation  into  the  cold  U-tube, 
the  willemite  in  the  cold  tube  suddenly  shines  out 
brilliantly,  at  the  point  where  the  emanation  con- 
denses. 

So  long  as  the  U-tube  is  kept  in  the  liquid  air 
the  emanation  will  remain  there,  though  I  continue 
to  send  a  gentle  blast  of  air  from  the  bellows.  But 
a  few  moments  after  taking  the  tube  out  of  the 
liquid  air,  it  warms  up  to  the  point  (-150°  Centi- 
grade) at  which  the  emanation  again  resumes  its 
gaseous  form,  and  now  we  can  blow  it  out  with 
a  single  puff  of  air.  See  !  I  blow  it  out  through 
the  narrow  tubing,  which  I  have  connected  to  the 

U-tube,   into   a  large  flask  dusted   over  its   inside 

i 


ii4  THE  RADIUM  EMANATION 

surface  with  the  phosphorescent  sulphide  of  zinc. 
In  the  dark  the  globe  shines  out  with  a  soft  white 
light  like  some  fairy  lantern,  and  I  can  see  to  read 
my  watch  by  its  light.  The  physiological  effects 
of  the  radium  emanation  are  imperfectly  investi- 
gated and  are  probably  potent.  This  is  a  field  of 
investigation  I  personally  have  no  desire  to  explore, 
so  that  we  must  not  forget  to  cork  the  globe  and  so 
prevent  the  emanation  from  diffusing  out  into  the 
air  of  the  room. 

After  this  demonstration  you  may  have  some 
difficulty  in  really  believing  that  the  actual  amount 
of  gaseous  emanation  which  has  produced  these 
beautiful  effects  is  almost  infinitesimal.  By  making 
use  of  the  same  property — its  condensation  by 
liquid  air — the  actual  volume  occupied  by  the 
radium  emanation  freed  by  freezing  from  all  other 
gases  was  measured  by  Sir  William  Ramsay  and 
myself.  Imagine  a  bubble  of  air  the  volume  of  a 
good-sized  pin's  head,  say,  one  cubic  millimetre,  or 
one  fifteen-thousandth  part  of  a  cubic  inch.  It 
would  require  thirty  times  more  emanation  than  was 
actually  employed  in  the  last  experiment  to  fill 
a  bubble  of  this  size.  Of  course,  in  the  experiments 
this  small  quantity  of  emanation  was  mixed  with 
a  considerable  volume  of  air  for  convenience  in 
manipulation. 


ITS  INFINITESIMAL    QUANTITY  115 

It  requires  a  distinct  step  for  the  mind  to  assimi- 
late the  important  fact  that  the  property  of  radio- 
activity, which  so  far  has  been  studied  only  in  solid 
substances  and  minerals,  could  be  shown  equally 
by  a  gas,  and  this  fact  accounted  for  the  true 
nature  of  the  emanation  remaining  largely  unrecog- 
nised even  after  the  conclusive  experiment  I  have 
shown  you.  There  is,  of  course,  nothing  contrary 
to  the  nature  of  radioactivity  in  the  fact  that  it  is 
shown  by  a  gas.  When  we  apply  Mme.  Curie's 
theory  that  radioactivity  is  an  intrinsic  property 
of  the  atom,  and  of  the  element  in  question,  the 
difficulty  is  not  that  the  emanation  is  a  gas,  for 
many  elements  are  gases,  but  how  it  is  that  a  new 
radioactive  element,  such  as  the  emanation  un- 
doubtedly is,  should  result  when  radium  compounds 
are  dissolved  in  water,  and  this  question  we  have 
purposely  deferred. 

The  emanation,  as  we  have  employed  it  in  our 
experiments,  is  mixed  with  ordinary  air,  and  in  this 
way  it  can  be  dealt  with  and  treated  like  any  other 
gas.  We  have  blown  it  through  tubes  from  one 
end  of  the  lecture  table  to  the  other.  If  it  had 
been  an  ordinary  gas,  like  air,  no  one  could  have 
seen  it,  or  known  what  became  of  it.  But  being 
intensely  radioactive,  although  its  actual  quantity 
is  almost  inconceivably  small,  the  radioactivity 


n6    CHEMICAL  NATURE  OF  THE  EMANATION 

serves  as  a  sufficient  evidence  of  its  presence  or 
absence,  making  it,  as  a  matter  of  fact,  far  easier 
to  work  with  and  to  investigate  than  an  ordinary 
gas  in  ordinary  quantity.  If  a  mining  engineer 
wished  to  know  how  the  air  he  pumped  into  his 
mine  got  distributed  among  the  various  shafts  and 
pits,  he  could  not  do  better  than  to  put  a  little 
radium  emanation  into  the  entering  air,  and  then 
subsequently  to  take  samples  at  various  parts  of 
the  mine,  and  have  them  tested  for  content  of 
radium  emanation  by  a  gold-leaf  electroscope. 
Many  other  practical  problems  in  the  flow  of  gases, 
which  are  difficult  to  solve  by  ordinary  methods, 
might  be  readily  solved  by  the  help  of  this  new  gas. 

It  has  even  been  found  possible  to  settle  the 
chemical  nature  of  this  new  gas,  and  to  place  it  in 
its  proper  family  of  elements  in  the  periodic  table. 
Almost  all  gases,  according  to  their  various  natures, 
are  absorbed  when  subjected  to  the  action  of 
various  chemical  reagents.  Thus  oxygen  is  ab- 
sorbed by  phosphorus,  hydrogen  by  heated  copper 
oxide,  nitrogen  by  heated  magnesium,  and  so  on. 
The  exceptions,  namely,  gases  which  are  not  ab- 
sorbed by  any  reagents  and  which  will  not  combine 
with  anything,  are  the  newly  discovered  gases  of 
Lord  Rayleigh  and  Sir  William  Ramsay — argon, 
helium,  neon,  etc. — which  exist  in  atmospheric  air. 
The  quantity  in  the  air  of  these  gases  is  extremely 


THE  ARGON  FAMIL  Y  117 

minute  except  in  the  single  case  of  argon,  which 

is  present  to  the   extent   of  one   per   cent.     The 

radium  emanation,  like  argon,  is  not  absorbed  by 

any  known  reagent,  and  does  not  appear  to  possess 

any  power  of  chemical    combination.     It  may  be 

passed  unchanged  through  absorbents,  or  subjected 

to  drastic  chemical   treatment  which  would  suffice 

to  absorb  every  known    gas   except  those  of  the 

argon   type,   and   therefore  we   say  the  emanation 

is  probably  an  element  of  the  same  family  nature 

as  the  argon  gases.     Like  them,   it  exists  in  the 

form  of  single  atoms — that  is,  its  molecule  is  mon- 

atomic.     Radium,  on  the  other  hand,  in  its  chemical 

nature  is  extremely  similar  to   barium,   strontium, 

and  calcium,  a  family  known  as  the  alkaline-earth 

elements.     None  other  of  the  argon  elements  or  the 

alkaline-earth  elements  are  radioactive,  and  yet  the 

:  elements  are  quite  normal  in  their  chemi- 

ies,  closely  resembling  ordinary  elements, 

associated  in  the  clearest  and  closest  way 

Dr  other  of  the  old  well-known  types  or 

Quite   recently,   by  using   quantities   of 

)ut  fifteen  times  as  great  as  those  used  to- 

>ur  experiments,  it  has  been  possible  to 

ugh  of  the  emanation  for  it  to  be  possible 

•aph  its  spectrum.     This  proves  to  be  a 

characteristic    bright-line    spectrum,    re- 

n  general  character   the  spectra  of  the 

n  gases. 


n8  ENERGY  EVOLVED  FROM  THE  EMANATION 

It  has  been  found  possible  to  obtain  some  idea  of 
the  density  of  the  emanation  of  radium,  and  there- 
fore of  the  weight  of  its  atom,  from  experiments  on 
the  rate  of  its  diffusion  from  one  place  to  another. 
These  indicate  that  the  gas  is  extremely  dense — 
denser  probably  than  mercury  vapour — and  there- 
fore that  it  has  a  very  heavy  atom.  Finally,  by 
means  of  a  new  special  micro-balance  thousands  of 
times  more  sensitive  than  the  most  delicately 
constructed  chemist's  balance,  the  emanation  has 
actually  been  weighed  by  Sir  William  Ramsay  and 
Mr.  Whytlaw-Gray.  These  experiments  and  the 
whole  of  the  available  evidence  agree  in  indicating 
that  the  atomic  weight  of  the  emanation  is  222, 
which  is  four  units  below  that  of  radium,  and  there- 
fore is  the  fourth  heaviest  known. 

The  heat  given  out  by  a  gram  of  radium,  as  we 
have  seen,  is  133  calories  per  hour,  but  it  must  be 
understood  that  this  refers  to  radium  in  its  normal 
condition  containing  its  full  quota  of  emanation. 
After  solution  in  water,  that  is,  after  the  emanation 
is  extracted,  the  radium  gives  out  heat  to  the 
extent  of  only  thirty-three  calories  per  hour,  while 
the  emanation  produces  one  hundred  calories  per 
hour.  That  is  to  say,  the  emanation  of  radium 
gives  three  times  as  much  energy  as  the  radium 
from  which  it  is  derived,  although  the  actual  amount 


ENERGY  EVOLVED  FROM  THE  EMANATION  119 

of   matter    in    the    emanation    is    itself  practically 
imperceptible. 

Now,  perhaps  it  is  easy  to  understand  how  it  is 
that  the  minuteness  of  the  quantities  of  material 
offers  no  barrier  in  the  investigation  of  radioactivity. 
Mass  is  not  the  only  consideration.  A  very  small 
bullet  suffices  to  work  terrible  havoc,  in  spite  of  its 
smallness,  by  means  of  the  kinetic  energy  with 
which  it  is  impelled.  A  little  torpedo,  stuffed  full 
of  imprisoned  energy  in  the  form  of  explosives, 
suffices  to  sink  an  enormous  battleship.  A  quantity 
of  emanation,  which  certainly  does  not  weigh  a 
hundred-thousandth  part  of  a  grain,  gives  out  enough 
energy  to  produce  effects  plainly  visible  to  you  all 
at  the  very  back  of  the  room. 

If,  instead  of  the  thirtieth  part  of  a  pin's  head 
full,  we  could  obtain  a  pint  of  this  gas — and  to 
obtain  such  a  quantity  half  a  ton  of  pure  radium 
would  be  required — it  would  radiate  the  energy 
of  a  hundred  powerful  arc-lamps.  Indeed,  as 
Rutherford  has  said,  no  vessel  would  hold  it. 
Such  a  quantity  would  instantly  melt  and  dispel  in 
vapour  any  material  known. 

These  new  facts,  which  transpire  the  moment  we 
begin  to  make  a  systematic  investigation  of  the 
radioactivity  of  radium,  make  the  second  alterna- 


i2o  THE  DECAY  OF  THE  EMANATION 

tive,  that  the  energy  of  radium  is  derived  from 
outside,  well-nigh  incredible.  For  to  account  for 
the  energy  evolved  from  the  emanation  we  must 
suppose  all  space  to  be  everywhere  traversed  by 
new  and  mysterious  forms  of  radiant  energy  of 
such  tremendous  and  incredible  power  that  the 
explanation  is  harder  to  believe  than  the  fact  it 
is  supposed  to  explain.  To  avoid  the  necessity 
of  supposing  that  the  energy  resides  within  the 
comparatively  small  amounts  of  radioactive  matter 
in  existence,  we  must  fill  the  whole  of  external  space 
with  radiant  energy  of  a  similar  order  of  magnitude. 
This  is  straining  at  a  gnat  and  swallowing  a  camel. 

Fortunately  there  is  a  crucial  test  by  which  we 
are  now  in  a  position  to  decide  between  the  two 
alternative  views.  Let  us  apply  the  theorem  we 
have  already  deduced  (p.  96)  from  general  princi- 
ples. If  the  energy  comes  from  within  the  radio- 
active matter,  its  radioactivity  must  in  course  of 
time  diminish  and  decay — the  more  rapidly  the 
more  powerfully  radioactive  it  is.  Whereas,  if  the 
energy  comes  from  the  outside,  however  powerful  the 
radioactivity  may  be,  there  is  no  reason  why  it  should 
not  continue  indefinitely  with  undiminished  power. 

We  have  seen  that  the  emanation  is,  mass  for 
mass,  far  more  intensely  radioactive  even  than 
radium,  and,  if  the  energy  comes  from  within,  it 
is  to  be  expected  that  the  activity  of  the  emana- 
tion will  be  short-lived  in  comparison  with  that 


THE   DECAY  OF  THE  EMANATION          121 

of  radium,  whereas,  if  the  energy  is  derived  from 
outside,  no  such  decay  is  to  be  anticipated.  Does 
the  radioactivity  of  the  radium  emanation  diminish 
or  decay,  or  does  it  continue  permanently  ? 

The  answer  to  this  question  is  that  the  radio- 
activity of  the  emanation  rapidly  decays  away  from 
day  to  day.  Four  days  hence  the  activity  will  be 
but  one-half  of  what  it  now  is.  In  eight  days  the 
activity  will  be  reduced  to  one-fourth,  in  twelve 
days  to  one-eighth,  in  sixteen  days  to  one-sixteenth, 
and  so  on,  diminishing  practically  to  zero  at  the  end 
of  a  month  in  a  descending  geometrical  progression 
with  the  lapse  of  time. 

The  light  from  the  glowing  willemite  in  this 
tube,  when  it  is  left  entirely  to  itself,  will  gradually 
fade,  and  at  the  end  of  a  month  will  have  died 
almost  completely.  Vast  as  is  the  store  of  energy  in 
matter  which  is  released  in  the  radioactive  process, 
it  is  not  infinite,  and  in  the  radium  emanation  we 
have  an  example  of  a  change  proceeding  so  rapidly 
that  only  a  few  weeks  are  necessary  for  its  com- 
pletion. 

Haifa  mystery  is  usually  greater  than  the  whole, 
and  in  science  when  mysteries  begin  to  appear  on 
all  sides,  the  explanation  is  often  near  at  hand. 
We  dissolved  a  compound  of  radium  in  water,  and 
the  greater  part  of  its  activity  disappeared  in  the 
process.  Then  little  by  little  the  lost  activity  was 


122      THE  REPRODUCTION  OF  THE  EMANATION 

spontaneously  recovered,  and  at  the  end  of  a  month 
the  radium  was  not  appreciably  less  active  than  at 
first.  The  disappearance  of  the  greater  part  of  the 
activity  after  solution  was  explained  by  the  fact  that 
an  extremely  radioactive  gas — the  emanation — was 
liberated  during  the  act  of  solution,  and  this  carried 
away  with  it  the  whole  of  the  radioactivity  which 
the  radium  had  lost.  But,  lo !  while  the  radium 
slowly  recovered  its  original  radioactivity,  the 
emanation  lost  what  it  had  at  first  possessed.  A 
quantitative  examination  of  these  two  processes  of 
decay  and  recovery  at  once  showed  that  the  total 
radioactivity  had  not  been  affected,  but  had  re- 
mained constant  in  spite  of  the  treatment  to  which 
the  radium  had  been  subjected.  This  is  a  funda- 
mental law  of  universal  application  to  all  radioactive 
bodies,  and  it  has  been  called  the  Law  of  the  Con- 
servation of  Radioactivity.  Whatever  you  do  to 
any  radioactive  substance  you  cannot  artificially 
alter  the  total  radioactivity,  though  you  may 
frequently,  as  in  this  example,  divide  it  into  several 
parts,  for  reasons  that  will  soon  be  clear. 

It  is  easy  enough  on  the  first  alternative  to 
account  for  the  comparatively  rapid  decay  of  the 
activity  of  the  emanation  of  radium.  It  is  dissipat- 
ing its  internal  store  of  energy  so  rapidly  that  it  is 
soon  exhausted.  It  is  a  clear  case  of  a  short  life 
and  a  merry  one.  But  how  is  the  gradual  recovery 
of  the  radioactivity  of  the  radium  in  the  course  of 


THE  REPRODUCTION  OF  THE  EMANATION    123 

time  to  be  explained  ?  This  is  the  key  to  the 
whole  problem,  and  on  the  second  alternative  no 
answer  whatever  can  be  given.  The  explanation 
that  the  energy  of  radioactive  substance  is  derived 
from  outside  is  not  merely  incredible.  It  is  alto- 
gether insufficient. 

Imagine  that  a  month  has  elapsed,  and  that  the 
radium,  which  has  now  recovered  completely  its 
lost  activity,  is  again  dissolved  in  water  and  evapo- 
rated down  to  dryness  exactly  as  before.  Again 
you  would  find  that  in  the  process  the  radium  had 
lost  the  same  large  proportion  of  its  radioactivity, 
and  again  you  would  obtain  from  it  a  new  amount  of 
emanation  no  less  than  that  which  is  on  the  table 
to-night.  Repeat  the  experiment  as  often  as  you 
please  and  you  will  find  the  result  always  the  same. 
While  the  emanation  you  separate  from  the  radium 
is  decaying  away  from  day  to  day,  a  fresh  crop  is 
being  spontaneously  manufactured  by  the  radium. 
The  change  of  the  radium  into  the  emanation  is,  as 
a  matter  of  fact,  only  the  first  of  a  long  series  of 
successive  changes  of  a  similar  character.  The 
gaseous  emanation  in  turn  rapidly  changes  into  a 
third  body,  not  a  gas,  called  Radium  A ;  this  into  a 
fourth,  called  Radium  B  ;  and  so  on.  Nine  suc- 
cessive changes  are  at  present  known,  which  we 
shall  have  to  give  some  account  of  later. 

This  explanation  of  radioactivity,  which  has  come 


i24  A  TOM  1C  DISINTEGRA  TION 

to  be  known  as  the  theory  of  atomic  disintegration, 
was  put  forward  by  Professor  Rutherford  and  my- 
self as  the  result  of  a  long  series  of  experimental 
investigations  carried  out  in  the  Macdonald  Physical 
and  Chemical  laboratories  at  M'Gill  University, 
Montreal.  It  has,  since,  not  only  shown  itself 
capable  of  interpreting  all  the  very  complicated 
known  facts  of  radioactivity,  but  also  of  predicting 
and  accounting  for  many  new  ones.  Although  on 
the  surface  a  revolutionary  addition  to  the  theories 
of  physical  science,  it  must  be  remembered  that  it 
is  the  facts  of  radioactivity  which  are  really  re- 
volutionary. While  accommodating  these  strange 
new  facts  the  disintegration  theory  conserves  in  a 
truly  remarkable  way  the  older  established  prin- 
ciples of  physical  science.  Without  such  a  guiding 
hypothesis,  reconciling  the  old  and  the  new,  it  is 
safe  to  say  that  the  facts  of  radioactivity  would 
ultimately  have  wrought  a  far  greater  change  in 
scientific  theory  than  has  actually  taken  place. 
Although  the  emanation  of  radium  is  not  and,  as 
we  shall  come  to  see,  never  can  be  obtained  in 
palpable  quantities — it  is  changing  too  rapidly  for 
that — we  know  almost  as  much  about  its  nature  and 
properties  as  we  do  about  any  of  the  older  gases. 

A  very  important  point  is  that  just  as  we  cannot 
really  alter  the  radioactivity  of  a  body  artificially  in 


RADIOACTIVE  EQUILIBRIUM  125 

any  way,  we  cannot  and  do  not  in  any  process  in- 
fluence the  rate  at  which  the  emanation  is  being 
formed  from  radium  or  the  rate  at  which  it  in  turn 
spontaneously  changes.  The  same  amount  is  always 
in  existence  whether  you  separate  it  or  not.  The 
apparent  constancy  of  the  radioactivity  of  radium 
is  not  the  real  constancy  to  be  expected  of  a  trans- 
forming mechanism.  It  is  the  apparent  constancy 
produced  by  the  equilibrium  between  continuous 
and  opposing  changes,  on  the  one  hand  the  rapid 
decay  of  the  part  of  the  radioactivity  due  to  the 
emanation,  and  on  the  other  the  regeneration  of 
fresh  emanation  as  fast  as  the  old  disappears. 
This  process  of  regeneration  is  always  going  on  at 
a  perfectly  definite  and  unalterable  rate,  and  the 
property  of  producing  a  certain  definite  amount  of 
emanation  in  a  given  time  is  as  much  a  part  and  parcel 
of  the  very  nature  of  radium — and  indeed  the  best 
and  most  easily  applied  qualitative  and  quantitative 
test  for  the  presence  of  radium  in  the  minutest 
quantity  that  we  possess — as  is  its  power  of 
giving  the  rays  which  lit  up  the  X-ray  screen  and 
discharged  the  silk  tassel,  or  as  its  power  of  gener- 
ating heat. 

All  of  these  properties  are  but  the  various 
aspects  of  a  single  primary  cause.  The  element 
radium  is  changing,  so  slowly  it  is  true,  that  at 


126     OLD  AND  NEW  METHODS   CONTRASTED 

first  sight  it  appears  not  to  be  changing  at  all, 
and  yet  with  so  tremendous  and  unparalleled  an 
evolution  of  energy  that  the  transformation  of  an 
otherwise  imperceptible  part  of  its  mass  is  accom- 
panied by  an  amount  of  energy  so  great  that  the 
change  could  not  by  any  possibility  have  remained 
unknown.  The  emanation  is  the  first  main  pro- 
duct of  the  change  of  radium.  If  the  emanation 
were  like  lead  or  any  ordinary  element  it  would 
take  years  of  accumulation  and  the  most  minute 
and  patient  investigation  to  detect  its  production. 
But  it  is  not.  The  emanation  changes  again  into 
a  third  type  of  matter  we  have  not  yet  considered 
(the  nature  of  which  does  not  yet  concern  us), 
but  whereas  it  would  take  hundreds  of  years  for 
any  appreciable  fraction  of  the  radium  itself  to 
change,  the  change  of  the  emanation  is  rapid  and 
goes  to  practical  completion  within  a  single  month. 
It  is  precisely  on  this  account  that  we  can  work 
with  and  detect  such  almost  infinitesimal  quantities. 
What  may  be  termed  the  material  evidence  of 
radioactive  change,  the  detection,  by  purely  chemi- 
cal or  spectroscopic  methods,  of  the  materials 
formed  in  the  changes,  is  still  scanty,  although  not 
altogether  lacking.  But  the  radioactive  evidence, 
which  depends  not  on  the  material  produced,  but 
upon  the  energy  evolved,  and  on  the  way  in  which 
the  energy  is  manifested,  is  abundant  and  sufifi- 


OLD   AND   NEW  METHODS   CONTRASTED     127 

cient.  So  long  as  the  energy  evolved  is  sufficient 
in  quantity,  and  of  a  kind  suitable  for  detection 
in  any  of  the  various  ways  I  have  illustrated,  the 
actual  quantity  of  matter  producing  the  energy  is 
of  no  consequence. 

But  the  amount  of  energy  produced  by  any 
change  depends  not  only  on  the  quantity  of  matter 
changing,  but  also  on  the  time  the  change  lasts, 
that  is,  on  the  period  of  life  of  the  changing  matter 
Chemical  and  spectroscopic  methods  of  detecting 
matter  depend  on  quantity,  whereas  radioactive 
methods  depend  on  quantity  divided  by  life.  The 
shorter  the  life  of  the  changing  substance  the  less 
of  it  is  necessary  for  its  detection  by  means  of  radio- 
activity. This  is  a  merely  preliminary  and  tenta- 
tive indication  of  the  operation  of  an  exactly  com- 
pensating principle  of  great  importance,  which  later 
it  will  be  possible  to  formulate  as  a  general  law. 
Its  result  in  the  long  run  is  this.  Each  of  the 
ephemeral  intermediate  substances  in  the  cascade 
of  changes  comes  equally  within  our  powers  of 
investigation,  whether  it  changes  slowly  or  rapidly, 
whether  it  lasts  long  enough  to  accumulate  in 
ponderable  quantity,  or  whether  it  is  changing  so 

rapidly  that  it 

anon, 

Like  snow  upon  the  desert's  dusty  face, 
Lighting  a  little  hour  or  two,  is  gone. 


CHAPTER    VI. 

The  connection  of  the  a-particle  with  radioactive  changes  —  The  a-par. 
ticle  and  helium  —  Accumulation  of  helium  in  geological  time- 
Discovery  of  helium  in  the  sun  and  on  the  earth—  Its  connection 
with  radioactivity  —  Production  of  helium  from  radium—  Its  pro- 
duction from  uranium  and  thorium—  Proof  that  the  a-particle  is 
an  atom  of  helium  —  The  nature  of  the  first  change  of  radium- 
Radioactive  "  recoil." 


E^ST  week  we  studied  the  first  step  in  the 
evidence  that  radium  is  changing,  and  con- 
sidered in  some  detail  the  chief  practical  reason  why 
such  changes  have  proved  within  our  powers  of 
discovery,  namely,  that  the  change  is  not  single  but 
proceeds  in  cascade  from  stage  to  stage,  producing 
ephemeral  intermediate  transition-forms,  of  which 
the  radium  emanation  is  one,  almost  inconceivably 
minute  in  their  actual  quantity  but  evolving  in  their 
next  change  very  large  amounts  of  energy,  by 
means  of  which  it  is  possible  to  trace  them  and 
study  their  nature  with  ease.  We  considered  the 
first  product  of  the  change  of  radium,  namely,  the 
emanation  of  radium,  its  nature  and  properties,  and 
its  continual  production  from  radium.  We  reserved 
purposely  the  examination  of  the  connection  between 
radium  and  the  emanation  it  produces.  Now  I  wish 
to  combine  with  the  knowledge  we  have  gained  of 

12S 


RADIOACTIVE    CHANGE  129 

the  nature  of  the  radium  emanation  that  already 
considered  (Chapters  III.  and  IV.)  with  reference  to 
the  nature  of  the  a-particle. 

A  radium  salt  is  dissolved  in  water,  and  the 
imprisoned  emanation,  which  was  formed  but  stored 
during  the  previous  month  throughout  the  whole 
mass  of  the  substance,  is  thereby  liberated  and 
escapes.  The  radium  left  to  itself  continues  to 
produce  fresh  emanation  at  a  steady  rate.  The 
released  stores  of  emanation  begin  to  lose  their 
radioactivity.  We  shall  confine  our  attention  at 
first  solely  to  the  case  of  the  radium. 

When  radium  in  this  way  is  freed  from  all  pre- 
viously formed  emanation  it  still  gives  out  a-par- 
ticles,  although  only  about  one-fourth  as  many  as  it 
gives  out  when  it  contains  its  full  quota  of  emana- 
tion and  other  products.  These  a-particles  we 
regard  as  produced  from  the  radium  atom  in  the 
same  change  as  that  in  which  ^ 

the  emanation  is  produced.    The          ^ 
emanation  is  regarded,  in  fact,  as 

radium      that      has      lost      One      a-       Radium.        Emanation. 
.   .  FIG.  20. 

particle. 

This,  which  is  a  perfectly  general  point  of  view, 
was  proved  five  years  ago  by  the  consideration  of  a 
mass  of  evidence  accumulated  with  reference  to  the 
similar  changes  going  on  in  the  element  thorium, 
but  much  of  this  lies  beyond  the  scope  of  the 


CPO 


130      HELIUM  AND  RADIOACTIVE    CHANGE 

present  course.  The  evidence  that  has  since  been 
accumulated  enables  the  same  deduction  to  be  more 
simply  made,  and  this  alone  need  be  considered. 
Henceforth  the  original  reasoning  as  to  the  nature 
of  atomic  disintegration,  although  it  was,  when  first 
put  forward,  very  complete  and  convincing  to  those 
acquainted  with  the  whole  of  the  experimental  facts, 
will  be  largely  replaced  by  the  more  direct  evidence 
since  obtained. 

We  have  seen  in  considering  the  nature  of  the 
a-rays  that  they  are  now  regarded  as  due  to  the 
flight  of  swarms  of  helium  atoms  expelled  from  the 
radioactive  substance  with  an  almost  inconceivable 
speed  of  from  8000  to  12,000  miles  per  second. 
Long  before  the  real  nature  of  the  a-particle  was 
known,  helium  had  been  first  predicted  to  be  and 
then  proved  experimentally  to  be  a  product  of  the 
radioactive  changes  of  radium,  and  this  chapter  in 
the  development  of  the  subject  has  something  more 
than  an  historical  interest. 

Before  proceeding,  one  underlying  consideration 
governing  the  view  that  an  atom  of  helium 
and  an  atom  of  emanation  are  simultaneously 
formed  when  an  atom  of  radium  disintegrates, 
must  be  made  clear.  It  refers  to  the  relative 
quantities  of  each  product,  helium  and  emana- 
tion, which  it  may  be  expected  will  be  formed  by 


RADIOACTIVE  EQUILIBRIUM  *3» 

the  continuous  operation  of  the  process.  Helium 
we  know  is  not  radioactive,  and  therefore  there  is 
no  evidence  that  helium  is  changing  in  any  way, 
and  we  may  in  this  sense  refer  to  it  as  one  of  the 
ultimate  products  of  the  change.  The  emanation, 
on  the  other  hand,  is  changing  so  rapidly  that  the 
change  may  be  regarded  as  complete  in  the  course 
of  a  single  month.  The  bodies  it  is  changing  into 
we  have  not  yet  dealt  with,  and  they  do  not 
immediately  concern  us. 

Now  a  changing  substance,  like  the  emanation, 
cannot  possibly  accumulate  in  quantity  with  lapse  of 
time  beyond  a  certain  very  small  extent.  It  is  true 
it  is  constantly  being  formed  from  radium  in  the 
same  way  as  helium,  but  whereas  the  helium,  being 
a  stable  substance,  may  be  expected  to  accumulate 
in  a  quantity  that  is  proportional  to  the  time 
that  elapses,  the  quantity  of  emanation  will  not 
increase  beyond  a  certain  point.  For  in  a  very 
short  time  after  the  process  of  accumulation  of 
emanation  from  the  radium  begins,  as  much  emana- 
tion will  itself  change  as  is  formed,  and  the  quantity 
from  that  time  on  will  remain  constant.  This 
condition  is  known  generally  as  "  radioactive 
equilibrium,"  and  when  we  speak  of  the  emanation 
being  in  equilibrium  with  the  radium  we  mean  that 
the  quantity  of  emanation  has  reached  a  maximum 
and  does  not  further  accumulate  with  further  lapse 


132  ACCUMULATION  OF  PRODUCTS 

of  time.  In  the  case  of  the  emanation  practical 
equilibrium  results  in  the  comparatively  short  time  of 
a  few  weeks.  That  is  to  say,  however  long  radium 
is  left  undisturbed  to  accumulate  its  emanation,  the 
quantity  of  the  latter  never  exceeds  a  practically 
almost  infinitesimal  one,  for  it  is  a  quantity  which  is 
produced  from  the  change  of  the  radium  in  quite 
a  short  period  of  time.  Its  quantity  is  therefore 
excessively  minute.  It  is  so  very  minute  that  were 
it  not  changing  and  evolving  energy  it  would  not  be 
detectable  by  any  ordinary  method. 

You  will  see  that  it  follows  at  once  from  this 
point  of  view  that  if  any  element  were  produced  in 
the  disintegration  of  radium,  which  itself  did  not 
change  but  was  permanent,  then  on  the  one  hand, 
owing  to  the  extreme  smallness  of  the  amount 
formed,  it  would  not  be  easy  in  a  short  period  to 
obtain  evidence  of  its  production,  by  means  of 
ordinary  chemical  tests,  but,  on  the  other  hand,  the 
quantity  would  go  on  accumulating  indefinitely 
with  lapse  of  time. 

As  we  saw  last  week,  the  first  evidence  of  atomic 
disintegration  was  dynamical  and  due  solely  to  the 
energy  which  is  evolved  in  the  process.  The 
answer  to  the  question  as  to  what  are  the  ultimate 
products  of  atomic  disintegration  must  be  looked  for 
on  quite  different  lines.  The  ultimate  products 
formed  will  be  too  small  for  detection  in  the  ordi- 


HELIUM  IN  MINERALS  133 

nary  way  by  the  statical  methods  of  chemistry  and 
physics,  but  they  will  accumulate  indefinitely. 

Since  the  processes  go  on  steadily,  so  far  as  we 
know,  in  the  minerals  in  which  the  radioactive 
elements  are  found,  the  ultimate  products,  formed 
through  past  ages  of  disintegration,  must  accumulate 
therein  from  one  geological  epoch  to  the  next.  So 
that  at  the  present  day  one  ought  to  find  in  the 
radioactive  minerals  the  ultimate  products  of  the 
disintegration  process,  accumulated  in  sufficient 
quantity  to  be  capable  of  detection  by  the  ordinary 
methods  of  chemistry. 

Now  the  radioactive  minerals  are  always  very 
complex,  and  contain  a  very  large  proportion  of  the 
total  number  of  elements  known,  so  that  in  most 
cases  it  is  impossible  to  deduce  very  much  from 
this  evidence.  Nevertheless,  there  was  one  clear 
definite  exception,  and  that  was  the  element 
helium. 

The  history  of  our  knowledge  of  this  element  is 
unsurpassed  by  that  of  any  other  in  interest.  Its 
very  name  (from  ^Xto?,  the  sun)  stands  witness  to 
the  fact  that  it  was  known  to  exist  in  the  sun  as  an 
element  before  it  was  known  to  exist  on  the  earth 
at  all.  It  was  discovered  in  1868  by  the  spectro- 
scope in  the  sun's  chromosphere,  by  means  of  the 
characteristic  bright  yellow  line  in  its  spectrum, 


134  PROPERTIES  OF  HELIUM 

which  is  technically  known  as  "  D3".  Then,  in 
1895,  Sir  William  Ramsay  discovered  it  in  certain 
minerals  found  in  the  earth's  crust,  and  made  a 
systematic  investigation  of  its  physical  and  chemical 
nature.  It  is  a  gas,  the  second  lightest  known, 
only  twice  as  dense  as  hydrogen,  and  for  long  was 
the  only  gas  which  successfully  resisted  all  efforts 
made  to  liquefy  it  by  extreme  cold  and  pressure.  In 
1908,  however,  Kammerlingh  Onnes  succeeded  by 
the  exercise  of  wonderful  experimental  skill  and 
persistence  in  reducing  helium  to  the  liquid  state, 
attaining  thereby  a  far  lower  temperature  (270° 
Centigrade,  or  only  3°  from  the  absolute  zero  of 
temperature)  than  has  ever  before  been  reached. 
It  is  readily  evolved  from  the  minerals  in  which  it 
is  found,  either  by  heating  them  or  by  dissolving 
them,  but  once  evolved  it  cannot  again  be  absorbed 
by  the  minerals  or  by  any  other  substance  known. 
Indeed,  helium  resembles  argon  perfectly  in  chemi- 
cal nature,  in  that  it  is  quite  without  any  combining 
power,  and  exists  free  as  single  atoms  without 
being  known  to  form  compounds  of  any  kind  what- 
ever. Its  atomic  weight  is  four  (hydrogen  =  i). 
Sir  William  Ramsay  drew  attention  to  the  fact 
that  all  the  minerals  in  which  he  found  helium 
contained  either  uranium  or  thorium.  This  was 
before  the  days  of  radioactivity,  and  for  long 
the  origin  of  the  helium — a  non-condensable,  non 


.HELIUM  AN  ULTIMATE  PRODUCT         135 

combining  gas — in  minerals  containing  uranium  and 
thorium  was  a  matter  for  comment  and  specula- 
tion. In  certain  cases  the  volume  of  helium 
evolved  is  nearly  a  hundred  times  as  great  as  the 
volume  of  the  mineral  in  which  it  is  contained. 

The  disintegration  theory  enabled  Professor 
Rutherford  and  myself  at  once  to  give  a  probable 
explanation  which  has  since  proved  to  be  correct. 
We  regarded  helium  as  one  of  the  ultimate  products 
of  the  disintegration  of  the  radioactive  elements, 
radium,  uranium,  and  thorium.  Forming  during 
the  long  ages  of  the  past  throughout  the  mass  of 
the  mineral,  which  is  often  of  a  glassy  nature,  it  is 
unable  to  escape  until  the  mineral  is  heated  or 
dissolved,  and  it  steadily  accumulates  with  the 
passage  of  geological  time.  We  ventured  to 
predict  that  helium  was  one  of  the  ultimate  pro- 
ducts of  radioactive  changes,  being  formed  in 
Nature  from  radium,  uranium,  and  thorium,  exces- 
sively slowly,  but  still  fast  enough  to  ensure  that  all 
minerals  containing  these  elements  must  contain 
helium  also.  This  has  since  been  proved  to  be 
the  case.  It  is  true  that  in  certain  uranium 
minerals — e.g.,  autunite  and  carnotite,  the  amount 
present  is  often  excessively  minute,  but  these  also 
are  just  the  minerals  which  it  is  believed  are  of 
extremely  recent  geological  formation. 


136     PRODUCTION  OF  HELIUM  FROM  RADIUM 

From  this  point  the  work  proceeded  along  two 
separate  lines.  Rutherford,  in  an  exhaustive  ex- 
amination of  the  nature  of  the  a-rays,  which  we 
have  already  considered,  proved  first  that  they  con- 
sisted of  positively  charged  atoms  expelled  with 
great  velocity.  At  first  their  mass  was  given  as 
twice  that  of  hydrogen,  on  the  assumption  they 
carried  one  atomic  charge.  Then,  as  the  sequel  to 
the  beautiful  counting  experiments  we  have  con- 
sidered, it  was  proved  in  1908  that  each  a-particle 
carries  two  atomic  charges  of  positive  electricity. 
Therefore  the  mass  of  the  a-particle  is  four,  that 
is  to  say,  it  is  the  same  as  that  of  the  atom  of 
helium.  This  made  it  very  probable,  therefore,  that 
the  a-particle  is  an  atom  of  helium. 

The  prediction  that  helium  was  a  product  of 
radioactive  changes  was  proved  directly  by  Sir 
William  Ramsay  and  myself  in  1903.  We  chose 
for  the  particular  case  of  radioactive  change  studied 
that  of  the  emanation  of  radium,  since  it  is  rapid, 
and  the  emanation  can  readily  be  obtained,  free 
from  other  gases,  first  by  the  action  of  suitable 
absorbents,  and  finally  by  condensing  it  with  liquid 
air  and  removing  the  gases  not  condensed  with  a 
pump.  So  purified,  it  was  sealed  up  in  a  small 
spectrum  tube,  so  that  the  spectrum  of  the  gas 
could  be  examined  at  will,  and  then  it  was  left  to 


FIG.  21.      Original  Spectrum-Tube  in  which  the  formation  of  Helium 
from  Radium  was  first  observed. 


Helium 


Hydrogen 


Red 


11! 


tv 


FIG.  22.  Dr.  Giesel's  Photograph  of  the  Spectrum  ot  the  Gas  from  Radium 
(II  20  minutes',  III  5  minutes'  exposure).  I  is  the  Spactrum  of  Helium, 
IV  that  of  Hydrogen,  for  comparison. 


To  face  p.  137. 


PRODUCTION  OF  HELIUM  FROM  RADIUM     137 

itself.     At  first  no  helium   was  present.     Helium, 
not  being  condensable  by  liquid  air,  could  not  have 
been  present  in  the  tube  as  first  prepared.     But  in 
the  course  of  three  or  four  days,  as  the  emanation 
disintegrated,    the    spectrum    of    helium    gradually 
made   its   appearance,  and   finally  the   whole  char- 
acteristic   spectrum    of   helium    was   given   by    the 
tube.      Fig.   21  shows  a  photograph  of  one  of  the 
original  spectrum  tubes  in  which  the  production  of 
helium  from  radium  was  proved.     This  observation 
of  the  production  of  the  element  helium  from  the 
radium  emanation,  and  therefore  (since  the  emana- 
tion in  turn  is  produced    from    radium)   from   the 
element  radium,  has  since  been  verified  and  con- 
firmed by  numerous  investigators  in  various  parts 
of  the  world.      It  has  also  been  found  by  Debierne 
in    a   similar    manner    by    the    spectroscope    that 
actinium,  a  radioactive  substance  found  by  him  in 
pitchblende,    produces    helium.       Dr.    Giesel    has 
actually  succeeded  in  photographing  the  spectrum 
of  the  gases  generated  by  radium,  and  one  of  his 
photographs  is  reproduced  in  Fig.    22.      It  repre- 
sents four  separate   spectra,   one   below   the   other 
in  parallel   strips.     The  uppermost  (I)  is  ordinary 
helium.     The   second  and   third  (II   and    III)  are 
two  photographs  obtained  from  the  gas  generated 
by  radium.     In  the  second  an  exposure  of  twenty 
minutes,    and    in    the    third    one    of    five    minutes 


i38     HELIUM  FROM  URANIUM  AND   THORIUM 

were  given.  The  lowest  spectrum  (IV)  is  that 
of  hydrogen.  It  will  be  seen  that  many  of  the 
helium  lines  are  present  in  the  spectrum  of  the 
gas  from  radium.  The  other  lines  are  those  of 
hydrogen,  due,  no  doubt,  to  the  presence  of  a 
trace  of  moisture.  The  figures  above  and  below 
the  plate  refer  to  the  stronger  lines  of  helium  and 
hydrogen  respectively  clearly  visible  in  photo- 
graph II.  They  refer  to  the  wave-lengths  in 
Angstrom  units  (io~10  metre).  It  must  be  re- 
membered that  the  (visually)  brilliant  yellow  line 
D3,  owing  to  its  colour,  appears  far  less  intense  in 
the  photograph  than  the  blue  and  violet  lines. 

I  have  been  engaged  for  four  years  in  an 
attempt  to  detect  the  production  of  helium  from 
the  primary  radio -elements  uranium  and  thorium, 
and  I  have  recently  succeeded  in  proving  in  both 
cases  that  helium  is  produced,  and,  moreover,  that 
the  rate  of  production  is  almost  exactly  what  is  to 
be  expected  from  the  theory  of  atomic  disintegra- 
tion. This  quantity  is  about  one  five-hundred- 
thousand-millionth  of  the  mass  of  the  uranium  or 
thorium  per  annum !  A  photograph  of  the  ap- 
paratus employed,  as  it  at  present  stands  in  the 
Physical  Chemistry  Laboratory,  is  shown  in  Fig.  23. 
These  are  seven  exactly  similar  arrangements 
side  by  side,  each  of  which  is  quite  separate  and 


pr 


P 


To  face  p.  138. 


HELIUM  FROM  URANIUM  AND   THORIUM     139 

unconnected  with  the  others,  Each  consists  essen- 
tially of  a  large  flask,  capable  of  holding  a  con- 
siderable quantity  of  the  material  experimented 
upon  in  the  form  of  solution.  Each  is  provided 
with  a  peculiar  form  of  mercury  tap,  which,  while  it 
serves  perfectly  to  keep  out  the  atmosphere  from 
the  flask  for  an  indefinite  time,  can  at  any  moment 
be  opened  by  sucking  down  the  mercury  in  the 
barometer  tubes,  so  that  the  accumulated  gases 
from  the  flask  can  be  extracted  and  tested  for 
helium  without  admitting  air.  Air  has  been  the 
great  trouble.  A  pin's-head-full  of  air  left  in  the 
whole  of  the  large  flask  or  in  the  solution,  or 
leaking  in  during  the  periods  of  accumulation, 
would  completely  ruin  the  experiment.  Most  of 
the  elaborations  of  the  apparatus  have  to  do  with 
the  preliminary  thorough  removal  of  the  air  from 
the  apparatus  before  the  experiments  are  com- 
menced. The  methods  of  testing  for  helium  are 
also  entirely  new.  They  depend  on  the  power  I 
found  was  possessed  by  the  metal  calcium,  when 
heated  to  a  very  high  temperature  in  a  vacuum,  of 
absorbing  the  last  traces  of  all  gases  except  the 
gases  of  the  helium  and  argon  type.  In  this  way 
the  minute  amount  of  helium  produced  (usually  not 
more  than  a  thousandth  part  of  a  cubic  millimetre) 
is  freed  perfectly  from  every  other  trace  of  gas  and 
water  vapour.  Finally,  it  is  compressed  by  means 


140  HELIUM  AND   THE  a  PARTICLE 

of  mercury  into  the  smallest-sized  spectrum  tube 
that  can  be  made  and  its  spectrum  examined.  As 
shown  in  numerous  special  experiments,  the  D8  line 
of  the  helium  spectrum  can  be  detected  with  cer- 
tainty if  one  millionth  part  of  a  cubic  centimetre, 
or  one  five-thousand-millionth  part  of  a  gram  of 
helium  is  present.  This  is  certainly  the  smallest 
quantity  of  any  element  that  has  ever  been 
detected  by  the  spectroscope. 

By  frequently  repeated  experiments  one  can  find 
for  each  flask  a  period  of  accumulation  that  must 
be  allowed  before  helium  can  be  detected  in  the 
expelled  gases,  and  so  one  can  obtain  a  measure  of 
the  rate  of  production  of  helium.  In  this  way 
I  have  obtained  helium  repeatedly  from  both 
uranium  and  thorium  salts,  and  the  rate  of  produc- 
tion, though  the  measurements  are  not  yet  finished, 
has  been  found  to  be  of  the  same  order  as  that  pre- 
viously calculated  from  the  disintegration  theory. 
For  the  case  of  uranium  the  rate  of  production  is 
about  two  milligrams  of  helium  from  a  thousand 
tons  of  uranium  per  year. 

The  position  is  then  this.  Helium  has  actually 
been  found  to  be  produced  from  the  various  radio- 
active substances  —  radium,  thorium,  uranium, 
actinium — which  have  in  common  the  fact  that  they 
all  expel  a-particles.  The  mass  of  these  particles 


HELIUM  AND    THE   a.  PARTICLE  141 

has  been  measured  and  found  to  agree  with  the 
mass  of  the  helium  atom.  All  a-particles  have 
been  proved  to  have  the  same  mass  and  to  differ 
only  in  the  initial  velocity  of  expulsion,  whether 
expelled  from  radium  itself,  from  the  emanation, 
from  actinium,  uranium,  thorium,  or  any  other  of 
the  bodies  which  expel  them.  Hence  we  are 
justified  in  concluding  that  the  a-particle  is  an  atom 
of  helium,  or  at  least  becomes  one  after  the 
velocity  with  which  it  is  expelled  is  lost  and  it  is 
brought  to  comparative  rest. 

One  further  step  in  this  long  converging  series 
of  experiments  clinches  the  argument.  We  have 
seeu  that  the  a-particle,  though  but  feebly  penetrat- 
ing, has  a  very  definite  small  penetrating  power. 
Now  glass  is  a  substance  that  can  be  blown  to  an 
excessive  degree  of  thinness  and  yet  retain  to  the 
full  its  air-tight  properties.  I  have  succeeded  in 
blowing  small  windows  of  glass  thin  enough  to 
allow  the  a-particle  to  get  through,  and  yet  strong 
enough  and  tight  enough  to  stand  the  pressure  of 
the  air  on  one  side  when  there  was  an  almost 
perfect  vacuum  on  the  other.  So  that  it  ought  to 
be  possible,  if  the  a-particle  is  an  atom  of  helium, 
by  storing  the  radioactive  substance  in  a  very  thin- 
walled  air-tight  glass  vessel,  to  get  helium  produced 
outside  the  vessel,  although  no  helium  or  other  gas 
in  the  ordinary  state  confined  inside  the  vessel 


142    ATOMIC   WEIGHT  OF  THE  EMANATION 

could  escape.  This  experiment  has  been  per- 
formed by  Rutherford  and  Royds  with  a  large 
quantity  of  radium  loaned  by  the  Austrian  Govern- 
ment. The  emanation  from  the  radium,  which 
gives  a-particles  and  has  been  shown  to  give 
helium,  was  stored  in  an  excessively  thin-walled 
but  still  perfectly  gas-tight  tube,  enclosed  within 
a  wider  vessel.  After  some  days  the  gas  in  the 
outer  vessel  was  found  to  contain  helium.  It 
was  proved  that  when  helium  was  stored  in  the 
inner  tube,  none  got  through  into  the  outer  vessel. 
This  final  experiment  clinches  the  proof  that  the 
a-particle  is  an  atom  of  helium. 

So  we  are  justified  in  writing  the  first  disintegra- 
tion suffered  by  radium  :— 


Radium.     Emanation.   Helium 
FIG.  24. 

There  is  a  great  deal  of  evidence  which  proves 
that  one  atom  of  a  radioactive  body  expels  but  one 
a-particle  at  each  disintegration.  Hence,  since  the 
atomic  weight  of  radium  is  226,  and  that  of  helium 
4,  the  atomic  weight  of  the  emanation  is  pre- 
sumably 222.  This  is  the  value  obtained  by  direct 
experiment  (Chapter  V.). 


RADIOACTIVE  RECOIL  143 

The  above  diagram  is  typical  of  no  less  than 
nineteen  different  radioactive  changes,  in  all  of 
which  an  atom  of  mass  between  240  and  200  expels 
an  a-particle,  or  helium  atom,  of  mass  50  or  60 
times  less.  By  the  usual  dynamical  law  it  is  to  be 
expected  that  the  heavy  residue  of  the  original  atom, 
whatever  it  is,  should  recoil  in  the  direction  opposite 
to  that  in  which  the  a-particle  is  expelled  with  a 
velocity  between  50  and  60  times  less  than  the 
a-particle,  that  is  to  say,  with  a  velocity  between 
150  and  250  miles  a  second.  The  kinetic  energy 
of  this  recoiling  atom,  since  it  depends  upon  the 
mass  multiplied  by  the  square  of  the  velocity,  will 
also  be  between  50  and  60  times  less  than  that  of 
the  a-particle.  The  velocity  and  kinetic  energy 
possessed  by  a  recoiling  atom,  though  greatly  inferior 
to  that  of  an  a-particle,  are  nevertheless  greatly 
superior  to  that  possessed  by  an  ordinary  gas 
molecule  at  any  attainable  temperature. 

The  phenomenon  of  radioactive  recoil  comes  into 
evidence  in  a  very  curious  and  interesting  manner, 
which  at  the  same  time  has  proved  of  very  great 
practical  utility.  Very  many  of  the  products 
resulting  from  the  expulsion  of  a-rays,  although 
after  their  formation  they  are  either  not  at  all 
volatile  or  can  only  be  volatilised  at  a  high  tem- 
perature, yet  at  the  moment  of  production  behave 
like  volatile  substances,  and  are  carried  away  under 


144  RADIOACTIVE  RECOIL 

suitable  circumstances  from  the  preparation  in  which 
they  are  produced,  and  deposited  on  the  nearest 
available  surface.  The  best  conditions  are  obtained 
by  working  in  a  good  vacuum,  and  charging  the 
preparation  positively,  and  the  surface,  on  which  it 
is  required  to  deposit  the  recoil  product,  negatively. 
The  residual  atom,  after  the  a-particle  is  expelled, 
carries  a  positive  charge,  and  so  is  attracted  to  the 
negatively  charged  surface.  It  is  essential  that  the 
preparation  should  be  in  the  form  of  a  very  thin 
layer  in  order  to  give  the  recoiling  product  a  chance 
of  escaping  from  it.  In  this  way  many  products,  of 
period  of  life  too  short  to  allow  of  their  being 
separated  by  any  other  method,  have  been  isolated 
and  identified  with  ease. 


CHAPTER   VII. 

A.tomic  disintegration  and  the  periodic  law — Questions  of  nomen- 
clature— Definition  of  the  chemist's  atom — Difference  between 
atoms  and  chemical  compounds — The  insufficiency  of  chemical 
methods  in  many  radioactive  problems — Relation  of  radioactivity 
to  the  electrical  theory  of  matter — Hypotheses  or  mental 
pictures — The  two  possible  pictures  of  atomic  disintegration — 
Sudden  explosive  character  of  the  disintegration — Law  of  radio- 
active changes — Chance  of  disintegration — Average  life  of  a  dis- 
integrating atom — Its  expectation  of  life — The  "how,"  not  the 
"  why,"  of  atomic  disintegration  explained — Determination  of  the 
period  of  average  life  of  atoms— Primary  radio-elements  and 
ephemeral  transition-forms  —  Radioactive  equilibrium  —  Average 
life  of  radium — The  total  energy  evolved  in  the  complete  disinte- 
gration of  radium. 

THE  question,  How  can  an  element  or  the  atom 
of  an  element  change,  has  given  rise  to 
many  arguments,  of  etymological  rather  than 
scientific  importance.  What  we  now  certainly  know, 
and  what  radioactivity  has  given  us  for  the  first 
time  the  opportunity  of  learning  is,  first,  that  some 
elements  do  change,  and  secondly,  koiu  they 
change.  The  element  radium  changes,  by  the  loss 
of  an  atom  of  helium,  into  the  emanation,  which 
is  about  as  different  from  radium  in  its  chemical  or 
material  nature  as  two  elements  well  could  be. 
The  one  is  a  member  of  the  group  of  alkaline- 
earth,  the  other  of  the  argon  family  of  elements. 

L  145 


J46  ATOMIC  DISINTEGRATION 

After  all,  is  not  this  rather  to  be  anticipated  ? 
When  we  arrange  the  elements  in  order  of 
their  atomic  weights — an  arrangement  which  led 
to  the  recognition  of  what  is  known  as  the 
Periodic  Law — the  most  sudden  and  surprising 
differences  appear  between  succeeding  elements. 
Chlorine,  potassium,  and  argon  are  three  succeed- 
ing elements  in  such  an  arrangement,  and  there  is 
no  resemblance  whatever  between  them.  In  the 
nine  successive  transformations  radium  undergoes, 
the  atom  suffers,  in  most  but  not  in  all,  a  disintegra- 
tion in  which  a  helium  atom  is  expelled.  The 
heavy  residues  of  the  original  atom  remaining  after 
the  successive  loss  of  one,  two,  three  and  so  on 
of  these  helium  atoms  constitute  the  intermediate 
bodies — the  emanation,  Radium  A,  Radium  B,  and 
Radium  C — successively  produced,  each  from  the 
preceding.  It  is  therefore  rather  to  be  expected 
that  the  succeeding  transition-substances  produced 
one  after  the  other  should  differ  entirely  from  one 
another  in  their  material  characteristics,  and  this, 
so  far  as  we  have  been  able  to  discover  them,  we 
find  to  be  fully  borne  out. 

Let  us  from  the  point  we  have  gained  now  face 
the  question,  which  has  proved  a  difficulty  to  so 
many,  of  how  it  is  we  find  that  the  elements  and 
the  atoms  are  actually  changing.  The  word  atom 


QUESTIONS   OF  NOMENCLATURE  14? 

is,  of  course,  derived  from  the  Greek,  and  at  first 
meant  the  indivisible  or  the  undivided.  For  a  long 
time  it  had  a  subjective  meaning  only,  being  the 
smallest  particle  imaginable,  rather  than  the 
smallest  particle  obtainable,  and  as  such  it  belongs 
to  metaphysics,  not  to  physical  science.  The  idea 
of  the  atom  was  first  given  an  objective  meaning 
by  Dalton.  He  showed  that  chemical  change  be- 
tween two  elements  occurs  in  definite  proportions 
by  weight  of  the  two  elements.  If  unit  weight  of 
one  is  taken,  the  weight  of  the  other  will  have  a 
definite  fixed  value.  But  often  the  same  two 
elements  unite  to  form  more  than  one  compound 
in  different  proportions.  Then,  if  unit  weight  of 
the  one  is  still  taken  for  reference  throughout,  the 
ratio  of  the  weights  of  the  other  in  various  com- 
pounds will  be  simple  multiples  or  submultiples 
of  one  another,  indicating  that  elements  do  not 
combine  in  haphazard  proportions,  but  "atom  for 
atom  "  by  fixed  increments  or  units  of  combination 
having  definite  relative  weight.  These  units  of 
chemical  combination  of  definite  relative  weight 
are  the  atoms  of  the  chemist.  In  all  the  various 
changes  of  matter  which  chemistry  has  investigated 
it  has  sufficed  to  regard  all  combination  as  taking 
place  atom  by  atom,  and  fractions  of  an  atom  or 
the  subdivision  of  atoms  has  not  been  necessary. 
In  compounds  the  component  atoms  preserve  their 


148     THE  ATOM  NOT  A    CHEMICAL    COMPOUND 

individuality  and  identity,  because  compounds  can 
always  be  decomposed  to  give  back  the  same 
elements  out  of  which  they  are  formed  and  not 
new  ones.  In  none  of  these  changes  does  any 
deep  change  of  the  component  atoms  themselves 
take  place.  As  chemical  changes  till  recently  were 
the  most  fundamental  material  changes  known,  the 
chemist's  atom  fulfilled  in  a  derived  sense  the 
ancient  meaning  of  the  smallest  particle  that  exists. 
It  did  not  suffer  subdivision  in  the  most  funda- 
mental changes  known.  But  in  this  sense  its  mean- 
ing was  coupled  with  that  of  the  particular  element 
to  which  it  referred.  Thus  the  atom  of  uranium 
is  about  240  times  as  massive  as  the  atom  of 
hydrogen.  An  atom  of  uranium  is  the  smallest 
particle  of  uranium  which  exists.  An  atom  240 
times  lighter  than  this  is  known,  but  it  is  not 
uranium,  it  is  hydrogen. 

The  discoveries  in  radioactivity  have  left  this 
meaning  of  the  word  atom  unchanged.  The 
atom  of  radium  is  the  smallest  particle  of  radium 
that  exists,  and  is  the  unit  of  all  the  chemical 
changes  radium  undergoes.  When,  by  new  and 
more  fundamental  changes  than  those  before 
known,  it  changes,  it  is  no  longer  an  atom  of 
radium.  The  matter  formed  is  as  unlike  radium  as 
any  body  well  could  be.  You  may,  if  you  like, 
regard  the  radium  atom  as  a  compound  of  the 


THE  ATOM  NOT  A    CHEMICAL   COMPOUND    149 

atom  of  emanation,  and  of  the  helium  atom  which 
result  on  its  disintegration,  as  it  certainly  is  such 
a  compound,  but  you  must  make  it  quite  clear  that 
you  do  not  mean  a  mere  chemical  compound,  which 
may  at  will  be  formed  from  and  decomposed  into 
its  constituents.  Were  radium  a  chemical  com- 
pound of  helium  it  would,  as  Sir  William  Huggins 
recently  pointed  out  to  me,  show  the  spectrum  of 
helium.  Instead,  it  shows  an  entirely  new  spectrum, 
clearly  analogous  to  but  distinct  from  that  shown 
by  barium,  its  nearest  chemical  relative.  The 
spectrum  of  helium  is  not  shown  until  after  the 
radium  has  disintegrated.  The  radium  spectrum 
does  not  contain  a  single  helium  line. 

The  most  vital  distinction,  however,  between 
an  element  and  a  compound  in  the  chemical  sense 
is  this.  Both  are  ultimately  compound.  Of  that 
there  can  be  now  no  doubt.  But  the  energy 
change  which  attends  the  resolution  of  an  element 
into  its  constituent  parts  is  of  an  order  of  a 
million  times  greater  than  in  the  case  of  the  resolu- 
tion of  any  chemical  compound.  Although  this  is 
a  question  of  degree,  it  is  of  a  degree  of  so 
entirely  different  an  order  of  magnitude  that  it 
completely  differentiates  the  two  types  of  com- 
plexes, and  nothing  but  confusion  can  result  from 
giving  to  each  the  same  name.  Radium  is  as  much 
an  element  as  any  of  the  other  eighty.  If  radium 


150  WORDS  AND  FACTS 

is  complex,  so,  almost  certainly,  are  all  to  greater 
or  less  degree.  If  radium  changes,  so  may 
(perhaps  even  so  do)  all.  Their  complexity  is  of 
a  completely  different  character  from  that  of  chemi- 
cal compounds,  and  it  is  best  in  the  end  to  retain 
the  old  words  "atom"  and  " element"  in  the 
sense  they  have  had  since  the  time  of  Dalton 
rather  than  attempt  to  meddle  with  this  tradi- 
tional, and  to  scientific  men,  well-understood 
nomenclature.  The  atom  of  the  chemist  remains 
exactly  what  it  was.  Why,  therefore,  alter  its 
name?  If  you  call  it  a  molecule,  how  are  you 
to  distinguish  it  from  the  chemical  molecule,  which 
has  also  its  own  definite  meaning  distinct  from  the 
chemical  atom  ? 

These  questions  of  nomenclature  at  first  diverted 
attention  from  the  experimental  fads,  and  gave  rise 
to  much  more  or  less  random  criticism  of  the  younger 
workers  in  radioactivity.  Another  source  of  con- 
fusion has  been  the  tendency  to  associate  the 
discoveries  in  radioactivity  with  other  entirely 
distinct  discoveries  made  somewhat  earlier  with 
reference  to  the  nature  of  the  negative  electron. 

It  was  thought  at  one  time  that  it  would  be 
possible  to  explain  the  atoms  of  matter  as  being 
built  up  entirely  of  electrons  or  atoms  of  electricity, 
which  is  as  little  in  accord  with  actual  evidence  as  it 


ELECTRICAL   THEORY  OF  MATTER          151 

would  be  to  regard  the  solar  system  as  composed 
entirely  of  planets  and  to  neglect  the  central  sun. 
The  problem  of  the  real  nature  of  the  atoms  of 
matter  has  not  been  completely  solved  by  either  of 
these  independent  scientific  advances. 

The  electrical  theory  of  matter  offers  a  possible 
view  of  what  the  atoms  might  be,  if — and  the  as- 
sumption is  not  a  small  one — two  kinds  of  electricity, 
the  unknown  hypothetical  positive  electricity  as  well 
as  the  experimentally  known  negative  electricity,  are 
assumed  to  exist.  Radioactivity,  on  the  other  hand, 
is  not  primarily  concerned  with  such  views,  but 
offers  new  and  definite  experimental  evidence  of 
what  the  atom  actually  is.  It  is  primarily  concerned 
with  matter  and  not  with  electricity,  and  it  tends 
rather  than  otherwise  to  reveal  the  extravagance  of 
the  assumptions  contained  in  the  view  that  matter 
is  constituted  entirely  out  of  electricity. 

Another  objection  to  the  validity  of  radioactive 
evidence  has  been  the  minuteness  of  the  amounts 
of  matter  on  which  the  evidence  is  based. 

It  has  been  stated  that  it  is  impossible  to  come  to 
any  settled  conclusions  in  regard  to  radioactivity, 
until  enough  of  the  materials  can  be  obtained  to 
suffice  for  the  requirements  of  chemical  investiga- 
tion. But  surely,  this  criticism  puts  weight  on  mere 
familiarity  with  the  older  methods  rather  than  on 
their  real  intrinsic  value.  The  tests  by  which  we 


152  MENTAL  PICTURES 

can  recognise  and  identify  with  ease,  and  measure 
with  accuracy  the  amount  of,  say,  one  billionth 
of  a  milligram  of  the  radium  emanation,  possess 
a  philosophical  foundation  which  would  challenge 
comparison  with  any  of  the  tests  of  the  chemist  on 
any  kind  of  matter,  in  any  quantity  great  or  small. 

It  is  my  intention  to  give  you,  so  far  as  I  am 
able  with  accuracy,  broad  general  mental  pictures 
of  radioactive  processes,  rather  than  the  detailed 
technical  investigations  on  which  these  pictures  are 
based.  Bear  in  mind  exactly  the  relation  of  such 
mental  pictures  to  the  discovered  facts.  The  pictures 
may  not  be  true,  but  they  are  not  demonstrably  false 
at  the  present  time.  That  is  to  say,  you  may  in  any 
case,  without  fear  of  being  led  into  error,  apply  the 
picture  you  have  to  what  is  taking  place,  and  the 
view  will  lead  you  to  expect  certain  consequences, 
and  these  consequences  in  every  known  case  agree 
with  the  facts.  Without  such  mental  pictures,  or 
generalising  hypotheses,  no  man  could  encompass 
even  a  small  part  of  one  science.  So  long  as  the 
deductions  from  the  hypothesis  are  in  agreement 
with  facts  and  can  be  used  to  predict  them  accu- 
rately, even  when  they  are  still  unknown,  thus 
saving  the  memory,  the  hypothesis  or  mental 
picture  is  not  even  supposed  or  expected  to  be 
the  absolute  truth.  So  long  as  all  the  known  facts 


NATURE   OF  ATOMIC  DISINTEGRATION       153 

occur  as  though  the  hypothesis  were  true,  the  latter 
serves  a  very  useful  purpose,  although  at  any  time 
it  may  be  replaced  by  a  deeper  view,  one  step 
nearer  to  absolute  truth. 

In  the  early  history  of  the  subject  two  possible 
alternatives  had  to  be  taken  into  account  with 
reference  to  the  exact  nature  of  radioactive  changes. 
Radioactivity  is  an  atomic  phenomenon,  and  the 
radio-elements  are  slowly  undergoing  changes. 
What  do  we  mean  by  "  slowly  "  in  this  connection  ? 
Two  possibilities  arise.  Either  the  slow  changes 
may  result  from  a  slow  gradual  alteration,  through 
all  the  atoms  of  a  radioactive  substance  gradually 
evolving  their  stores  of  internal  energy  and 
changing  by  slow  degrees  into  new  kinds  of  matter. 
This  point  of  view  it  was  never  possible  to  enter- 
tain for  a  moment.  Or,  the  change  is  slow  and 
gradual  with  regard  only  to  the  mass  of  the  sub- 
stance as  a  whole,  but  sudden  and  explosive  in 
character  with  regard  to  each  individual  atom  as  its 
turn  to  disintegrate  arrives.  This,  from  the  first, 
the  only  possible  point  of  view,  is  in  accordance 
with  all  that  has  since  been  discovered  with  regard 
to  the  nature  of  the  successive  disintegrations  and 
of  the  a-rays  expelled.  Radioactive  changes  pro- 
ceed in  cascade,  from  step  to  step,  the  accomplish- 
ment of  each  successive  step  taking  on  the  average 
a  definite  time.  But  as  regards  the  individual  atom 


154        NATURE   OF  ATOMIC  DISINTEGRATION 

disintegrating,  the  change  is  sudden  in  time  and  of 
the  nature  of  an  explosive  disruption,  in  which  an 
a-particle  is  expelled  with  enormous  speed,  and  the 
old  atom  becomes  ipso  facto  a  new  one,  of  atomic 
weight  four  units  less.  Regarding  the  individual 
radium  atom,  for  example,  there  is  no  gradual 
change  into  the  emanation  and  helium  atoms.  Re- 
garding the  whole  mass  of  radium,  there  is  a  very 
gradual  change  in  the  sense  that  some  definite 
small  proportion  of  the  whole  suffers  disintegration 
in  each  unit  of  time. 

This,  then,  is  the  very  vivid  mental  picture  of 
atomic  disintegration  which  the  detailed  researches 
in  radioactivity  have  established.  Any  one  radio- 
element  like  radium  being  considered  at  any  instant, 
among  its  innumerable  host  of  atoms,  most  of  which 
are  destined  to  last  for  hundreds,  some  for  thousands 
of  years,  a  comparatively  very  small  proportion 
every  second  fly  apart,  expelling  a-particles  and 
becoming  emanation  atoms.  Next  second  the  lot 
falls  to  a  fresh  set  to  disintegrate,  and  so  the  pro- 
cess goes  on,  a-particles  being  expelled  as  a  con- 
tinuous swarm,  and  yet  so  small  a  fraction  of  the 
whole  changing  that  the  main  part  of  the  radium 
will  remain  unchanged  even  after  hundreds  of 
years.  Now  consider  the  emanation  atoms  formed. 
These  are  much  less  stable  than  the  atoms  of 


THE   CHANCE   OP  DISINTEGRATION        155 

radium.  A  much  larger  fraction  of  these  disinte- 
grate every  second,  producing  more  a-particles  and 
a  new  body  not  yet  considered. 

It  is  now  necessary  to  consider  briefly  the  exact 
nature  of  radioactive  change  and  the  laws  it  follows. 
The  deduction  of  these  laws  is  a  matter  for  the  mathe- 
matician.   We  are  chiefly  concerned  with  the  general 
conclusions  which  have  transpired.     I  will  first  state 
the  most  important  of  these  in  words  divested  of 
mathematical  symbols.     The  chance  at  any  instant 
whether   any    atom    disintegrates    or    not    in    any 
particular  second  is  faced.      It  has   nothing  to  do 
with  any  external  or  internal  consideration  we  know 
of,  and  in  particular  it  is  not  increased  by  the  fact 
that  the  atom  has  already  survived  any  period  of 
past  time.     The  events  of  the  past  in  radioactive 
change  have,   so   far  as   we  can   tell,   no  influence 
whatever  on  the  progress  of  events  in  the  future. 
This    follows    from    the    consideration    of    the   one 
general  mathematical  law  which  all  known  cases  of 
atomic  disintegration  so  far  investigated  have  been 
found    to    follow.       Fortunately    the    law    itself    is 
simple.      Its  application  in  individual  cases  is  often 
complicated,    but    I    shall    confine    myself    to    the 
simplest,    which    are   at    the   same    time   the   most 
generally    important,    consequences.      The    chemist 
has  to  do  with  many  types  of  change  all  following 
different  laws.      In  some  the  rate  of  change,  that  is, 


156  LA  W  OF  RADIOACTIVE  CHANGE 

the  quantity  of  the  substance  changing  in  the  unit 
of  time,  is  proportional  to  the  quantity  of  the  sub- 
stance changing,  in  others  to  some  power  of  this 
quantity.      Now,  in  radioactive  change  the  rate  of 
change    is    invariably    simply    proportional    to    the 
quantity  of  changing  substance.     This  seems  easy 
enough,  but  I  would  warn  the  uninitiated  that  they 
must  not  overlook  the  important  fact  that  since  the 
quantity  of  a  changing  substance  itself  changes  as 
time  goes  on,  owing  to  the  progress  of  the  change, 
the  rate  of  change  being  proportional  to  the  quantity 
also  continuously  changes,   and  at  no  time  has  a 
constant    value.       Hence    you    cannot    get    much 
further    by    simple    arithmetic    and    algebra.      Of 
course,  in  the  case  of  a  slow  change  like  that  of 
radium  itself,  when  even  in  a  lifetime  the  quantity 
of  radium  is  not  very  appreciably  reduced  by  the 
operation  of  the  change,  it  is  allowable  to  neglect 
the  slow  alteration  of  the  rate  of  change  with  the 
time  and  to  consider  the  rate  of  change  as  con- 
stant, since  for  short  periods  of  time  it  essentially  is 
so.     In  most  cases  some  knowledge,  withal  a  slight 
one,   of  the    mathematics   of  continuously  varying 
quantities  is  essential  for  the  complete  deduction  of 
the  laws  of  radioactive  change.     However,  as  my 
intention  is  to  avoid  mathematics,    I    shall  simply 
state  these  consequences  ex  cathedra. 

The  rate  of  change  in  any  single  case  of  atomic 


THE  AVERAGE  LIFE  OF  AN  ATOM         157 

disintegration  is  proportional  to  the  quantity  of  the 
substance  which  is  changing.  The  usual  plan  is 
to  let  the  symbol  X  represent  the  fraction  of  the 
total  changing  per  second,  and  to  this  symbol  X  is 
given  the  special  name  "the  radioactive  constant." 
X  may  represent  a  small  or  a  large  fraction,  accord- 
ing to  the  particular  case,  according  as  the  dis- 
integration process  is  slow  or  rapid.  The  important 
point  is  that  it  is  a  real  constant  of  nature  in  every 
case,  independent  of  the  past  and  future  history  of 
the  substance,  its  actual  amount  whether  large  or 
small,  and  of  every  other  consideration  whatever. 
Thus  for  the  emanation  of  radium,  X,  the  radio- 
active constant,  has  the  value  1/481,250,  which 
signifies  that  in  this  case  i/,  481,250^1  of  the  total 
amount  of  emanation  in  existence  changes  per 
second.  The  next  step,  skipping  the  mathematics,1 
is  that  the  average  period  of  life  of  the  atom  of  a 
radioactive  substance,  that  is  to  say,  the  period  of 
time  in  seconds  it  exists  on  the  average  before  its 
turn  comes  to  disintegrate,  is  simply  the  reciprocal 
of  the  radioactive  constant,  or  i/X.  Thus  the 
average  life  of  the  radium  emanation  is  481,250 
seconds,  or  5*57  days. 

Now    as    radioactive    change    proceeds    during 
every    instant   at    the    rate    proportional    only    to 

1  So  far  as  I  know,  the  period  of  average  life  was  first  deduced  by 
Mr,  J.  K.  H.  Inglis,  to  whom  I  put  the  problem. 


158      THE  EXPECTATION  OF  FUTURE  LIFE 

the  total  quantity  of  substance  undergoing  the 
change,  which  is  present  and  remains  unchanged 
at  that  instant,  and  as  in  this  method  of  look- 
ing at  the  changes  we  do  not  consider  at  all 
the  absolute  quantities,  only  the  fraction  of  the 
whole  changing,  it  follows  that  X  is  always  of  the 
same  value  throughout  the  process  from  start  to 
finish.  It  also  follows  that  i/X,  the  period  of  average 
life  of  the  remaining  atoms,  does  not,  as  you  might 
be  inclined  to  suppose,  tend  to  lessen  as  time  goes 
on.  The  atoms  disintegrating  first  have  a  far 
shorter  period  of  life,  and  those  disintegrating  last 
have  a  far  longer  total  period  than  the  average.  But 
at  any  instant  throughout,  considering  only  the  atoms 
still  remaining  unchanged  at  that  instant,  then  from 
that  instant  the  average  period  of  life  is  always  i/X. 
Our  own  period  of  average  life,  of  course,  fol- 
lows very  different  and  far  more  complicated  laws. 
The  expectation  of  life  at  any  age  is  a  practical 
problem  for  the  actuary.  But  every  one  knows, 
owing  to  the  mortality  among  infants,  that  the  ex- 
pectation of  life  at  birth  is  less  than  shortly  after- 
wards, when  it  reaches  a  maximum  and  then  gets 
less  and  less  with  increasing  age.  The  "expectation 
of  life"  of  a  radioactive  atom  is  independent  of 
its  age — as  it  happens  the  simplest  possible  law  and 
one  lending  itself,  as  will  appear,  to  some  most 
beautiful  deductions. 


" WHY"  AND  "HOW"    OF  DISINTEGRATION     159 

This  answers  fully  the  general  question,  how 
does  an  element  change  ?  You  will  probably  wish 
to  know  why  it  changes  in  this  particular  way. 
That  cannot  be  said,  although  the  true  answer 
would  undoubtedly  take  us  far.  All  that  can  be 
stated  is  that  the  immediate  cause  of  atomic  dis- 
integration appears  to  be  due  to  chance.  If  the 
destroying  angel  selected  out  of  all  those  alive  on 
the  world  a  fixed  proportion  to  die  every  minute, 
independently  of  their  age,  whether  young  or  old,  if 
he  regarded  nothing  but  the  number  of  victims  and 
chose  purely  at  random  one  here,  and  one  there,  to 
make  up  the  required  number,  then  our  expectation 
of  life  would  be  that  of  the  radioactive  atoms.  This, 
of  course,  is  all  that  is  meant  by  the  statement  that 
the  course  of  atomic  disintegration  appears  to  be 
due  to  the  operation  of  "  chance." 

It  is  natural  to  inquire  why  this  particular  law  is 
followed.  On  this  fundamental  question  no  light  is 
yet  forthcoming.  There  is  always  "  a  cause  of  the 
ultimate  cause."  Atomic  disintegration  is  assuredly 
the  ultimate  cause  of  radioactivity.  It  does  not 
weaken  this  deduction  that  as  yet  we  have  not 
found  the  ultimate  cause  of  atomic  disintegration. 
Various  possible  causes  have  been  discussed.  Most 
of  them,  so  far  from  helping  the  elucidation  of  the 
"  why,"  do  not  conform  even  to  the  "how."  The  law 
of  radioactive  changes  shows  clearly  that  the  past 


160  THE  PERIOD   OF  HALF-CHANGE 

history  of  an  atom  does  not  increase  its  chances  of 
undergoing  disintegration  in  the  future,  which  is  a 
fundamental  step  gained,  although  it  leaves  the 
ultimate  problem  unsolved. 

There  is  another  way  of  stating  the  law  of  radio- 
active changes,  and  that  is  by  saying  that  as  the 
time  increases  in  arithmetical  progression  the 
amount  of  substance  remaining  decreases  in  geo- 
metrical progression.  Suppose  in  a  time  of  r 
seconds  one  half  of  the  total  amount  changes  and 
one  half  remains  unchanged.  In  the  next  period 
of  T  seconds,  2  r  altogether,  one  half  of  what  is 
left,  that  is,  one  quarter,  changes,  and  one  quarter  of 
the  total  remains  unchanged.  In  2  r  the  quantity  is 
reduced  to  i/22.  In  any  period  of  time  represented  by 
N  T  seconds,  where  N  is  any  multiple  or  submultiple, 
the  quantity  of  substance  remaining  is  i/2N.  It  remains 
to  state  what  relation  the  time  T  required  for  the 
half-change  to  occur,  bears  to  the  period  of  average 
life  i /A  of  the  former  way  of  considering  the  change. 
There  is  a  fixed  ratio  between  these  two  periods, 
the  latter  being  always  1*45  times  the  former.  In 
a  time  equal  to  the  period  of  average  life  i/X,  the 
quantity  of  substance  present  is  reduced  to  1/6  =  0*368 
of  the  initial  quantity. 

These  considerations  would  have  little  interest  to 
us  but  for  the  fact  that  they  afford  the  means  where- 
by the  period  of  average  life  of  any  radioactive 


EPHEMERAL  AND  PERMANENT  161 

element  can  by  their  aid  be  exactly  determined,  not 
only  for  those  transition-bodies  like  the  emanation, 
which  change  so  rapidly  that  we  can  watch  their 
complete  transformation  in  the  course  of  a  few 
days  or  weeks,  but  also  for  the  primary  radio- 
elements,  some  of  which  we  know  require  thousands 
of  millions  of  years  to  run  their  course  of  change. 
The  average  life  of  a  radioactive  element,  represent- 
ing as  it  does  a  fundamental  constant  of  nature,  is 
one  of  its  most  important  attributes.  Our  own 
period  of  average  life  being  strictly  limited,  it 
naturally  affects  very  much  our  way  of  looking  at 
the  various  radioactive  bodies.  If,  for  example, 
the  average  life  is  a  matter  of  a  few  days,  as  in  the 
case  of  the  radium  emanation,  we  regard  the  body 
as  an  ephemeral  transition-form.  If  it  is,  as  in  the 
case  of  radium,  a  few  thousand  years  we  are  in- 
clined to  look  upon  the  substance  as  a  permanent 
and  primary  radio-element.  There  is  really  not 
this  sharp  difference.  But  it  is  convenient  to 
divide  radioactive  bodies  into  two  classes,  and  in 
the  one  to  put  those  for  which  the  periods  of 
average  life  are  short  compared  to  our  own,  and  in 
the  other  to  put  those  for  which  the  periods  are 
long.  The  method  employed  to  determine  the 
value  of  this  fundamental  and  all-important  constant 
is  naturally  quite  different  in  the  two  cases.  In 


162  RADIO-ELEMENTS 

the  first,  simple  direct  observation  suffices.  Thus 
if  we  measure  the  decay  of  the  activity  of  any 
separated  quantity  of  the  emanation  of  radium 
with  time,  we  shall  find  that  it  decays  in  a  geo- 
metrical progression  with  the  time  to  half  its  initial 
value  in  the  course  of  3*84  days.  The  period  of 
average  life  is  1*45  times  greater,  or  5*57  days.  But 
in  the  case  of  a  body,  of  which  one  thousandth,  or 
one  thousand-millionth  as  the  case  may  be,  changes 
annually,  simple  direct  observation  does  not  help 
much.  How  are  we  to  proceed  ? 

In  the  first  place,  let  us  consider  the  cases  of 
uranium  and  radium.  We  may  determine  how 
many  times  more  powerfully  radioactive  radium 
is  than  uranium.  The  radioactivity  of  radium  is 
several  million  times  that  of  uranium  when  the 
a- rays  of  equal  quantities  of  the  two  elements  are 
compared.  From  this  it  may  be  concluded  that 
the  period  of  uranium  is  several  million  times  longer 
than  that  of  radium,  and  if  the  latter  is  known,  that 
of  uranium  may  be  roughly  estimated,  although  it  is 
a  period  of  some  thousands  of  millions  of  years. 

As  a  matter  of  fact,  there  is  a  very  beautiful 
generalisation,  I  have  already  referred  to  briefly, 
and  which  later  on  I  shall  try  to  develop  further 
by  the  aid  of  an  analogy,  by  means  of  which  the 
periods  of  average  life  of  the  radio -elements  of 
the  second  class,  those,  that  is,  which  are  long-lived 


AVERAGE  LIFE  OF  LONG-LIVED  ATOMS    163 

compared  with  ourselves,  have  come  into  the 
region  of  exactly  knowable  quantities.  If  the 
period  of  average  life  of  a  single  member  of  a 
series  of  successive  atomic  disintegrations  is  known 
the  others  can  be  calculated,  provided  certain  data, 
not  entirely  impossible  to  obtain,  are  known.  It 
will  clear  the  ground  considerably  if  I  attempt  to 
give  you  the  main  idea  succinctly  in  the  case  of 
radium  itself  and  of  the  first  product  of  its  dis- 
integration, the  emanation  of  radium.  I  have 
already  alluded  to  the  fact  that  owing  to  the  very 
rapid  disintegration  of  the  emanation  its  quantity 
does  not  continuously  accumulate,  but  reaches  an 
equilibrium  ratio  with  respect  to  the  radium  pro- 
ducing it,  in  which  the  amount  of  already  formed 
emanation  disappearing  is  exactly  counterbalanced 
by  the  amount  of  new  emanation  formed. 

This  state  of  things  is  known  generally  by  the 
name  of  radioactive  equilibrium.  The  importance 
of  the  existence  of  this  state  of  radioactive  equi- 
libriiim  it  is  impossible  to  overrate.  Many  prob- 
lems, as  we  shall  come  to  see,  which,  to  us  with 
our  limited  period  of  life,  might  well  appear  abso- 
lutely insoluble,  connected  as  they  are  with  periods 
of  time  so  vast  that  our  little  life  by  comparison 
appears  a  mere  moment,  are  solved  directly  by 
the  proper  application  of  this  principle.  Now  I 
am  only  giving  you  the  main  idea  and  one  specific 


1 64   AVERAGE  LIFE  OF  LONG-LIVED  ATOMS 

illustration  of  what  is  in  fact  a  law  of  great 
generality. 

By  the  law  of  radioactive  change,  if  AI  is  the 
radioactive  constant  of  radium,  i.e.  the  fraction  of 
the  whole  changing  per  second,  and  N  is  the  total 
number  of  radium  atoms  dealt  with,  then  the 
number  of  radium  atoms  changing  into  the  emana- 
tion per  second,  and  therefore  also  the  number 
of  atoms  of  fresh  emanation  produced  per  second, 
is  XjN.  But  in  equilibrium  this  equals  the  number 
of  emanation  atoms  disappearing.  If  the  radio- 
active constant  of  the  emanation  is  X2,  and  the 
number  of  atoms  of  emanation  present  during  equi- 
librium is  denoted  by  X,  the  number  of  emanation 
atoms  disappearing  per  second  is  X2X.  Hence  we 

have 

X     A. 

AXN  =  A2X  and  ^  =  <p • 

A£ 

This  law,  the  most  important  in  radioactivity, 
thus  states  that  in  successive  disintegrations  the 
product  accumulates  in  quantity  until  a  fixed  ratio 
with  respect  to  the  parent  body  is  attained,  and  this 
ratio  is  inversely  proportional  to  their  respective 
radioactive  constants  or  directly  proportional  to 
their  respective  average  lives.  It  is  necessary  for 
the  law  to  hold  true  that  the  period  of  the  parent 
body  should  be  much  longer  than  the  periods  of  any 
of  its  products,  and  in  this  case  the  product  selected 


AVERAGE  LIFE  OF  RADIUM  165 

need  not  necessarily  be  the  first  product,  but  may 
be  any  one  of  the  successive  products  formed  in  the 
series. 

X2  is  well  known  by  direct  observation.  Now  if 
X/N,  the  ratio  between  the  number  of  atoms  of 
emanation  and  of  radium  in  equilibrium  together, 
can  be  found,  then  X1}  the  radioactive  constant  and 
therefore  i/Ai,  the  period  of  average  life  of  radium 
can  be  deduced.  That  is  the  important  thing — the 
period  of  the  average  life  of  radium,  the  rate  at 
which  it  is  changing,  and  a  host  of  vitally  impor- 
tant consequences,  can  be  deduced.  For  a  slowly 
changing  body  like  radium  the  second  is  an  incon- 
veniently short  unit  of  time  to  employ  and  it  is 
better  to  take  a  year.  What  is  wanted  is  the 
fraction  of  any  quantity  of  radium  which  changes 
in  a  year.  The  quantity  X/N,  which  is  the  ratio 
of  the  number  of  atoms  of  emanation  and  of 
radium  in  equilibrium  together,  can  be  deduced  by 
ordinary  physico-chemical  laws  if  the  actual  volume 
of  emanation  in  equilibrium  with  a  given  quantity  of 
radium  can  be  determined.  As  already  mentioned 
(p.  114),  this  volume  was  first  measured  by  Sir 
William  Ramsay  and  myself  in  1904.  The  actual 
volume  of  emanation  is  excessively  minute,  but 
it  is  just  within  the  range  of  measurement.  From 
our  results  we  concluded  about  i/i  i5Oth  part  of  the 
radium  changes  annually,  so  that  the  period  of 


1 66  AVERAGE  LIFE  OF  RADIUM 

average  life  on  this  estimate  is  1 1 50  years.  Owing  to 
the  excessive  minuteness  of  the  volume,  the  method 
is  not  an  accurate  one,  tending,  since  the  volume 
of  emanation  is  likely  to  be  too  great  unless  every 
trace  of  other  gas  is  absent,  to  give  too  short  a 
period.  With  the  growth  of  the  subject  other 
methods,  less  direct  but  more  accurate,  have 
become  available.  Professor  Rutherford  recently, 
from  a  consideration  of  a  large  number  of  separate 
data  accumulated  by  himself  and  others  bearing  on 
this  question,  came  to  the  conclusion  that  the  period 
of  average  life  of  radium  is  not  very  far  removed  from 
2500  years,  and  we  shall  take  this  value  as  the  most 
probable.  It  may  suffer  slight  further  alteration 
as  fresh  data  are  accumulated,  but  it  is  very 
improbable  that  it  is  seriously  in  error.  Within 
narrow  limits  the  average  life  of  radium  may  be 
taken  to  be  2500  years. 

A  knowledge  of  this  important  constant  enables 
us  at  once  to  say  how  much  energy  any  quantity  of 
radium  would  evolve  in  the  course  of  its  complete 
change,  that  is,  during  a  period  of  some  thousands 
of  years.  We  saw  (p.  31)  that  a  gram  of  pure 
radium  evolved  about  133  calories  of  heat  per  hour. 
There  are  8760  hours  in  the  year,  so  that  in  a  year 
a  gram  of  radium  evolves  about  1,160,000  calories. 
In  a  year  i/25OOth  part  changes.  Therefore  in  the 
complete  change  of  one  gram  of  radium  no  less 


TOTAL  ENERGY  EVOLVED  BY  RADIUM      167 

than  2,900,000,000  calories  would  be  evolved.  The 
energy  evolved  in  the  change  of  radium  is  nearly 
a  million  times  greater  than  that  evolved  from  a 
similar  weight  of  matter  undergoing  any  change 
known  previously  to  the  discovery  of  radioactivity. 
By  the  burning  of  a  gram  of  coal,  for  example, 
only  about  8,000  calories  are  obtained.  In  this 
change,  however,  2§  grams  of  oxygen  are  also  con- 
sumed, so  that  per  gram  of  the  two  substances  taken 
together  the  heat  evolved  is  only  2,200  calories. 
On  this  basis  of  calculation  the  energy  of  radium  is 
well  over  a  million  times  that  furnished  from  the 
combustion  of  coal.  No  wonder  then  that  to 
account  for  the  boundless  energy  displayed  every- 
where in  the  starry  heavens  proved  a  difficult 
problem  for  physicists,  acquainted  with  no  more 
energetic  chemical  process  than  the  burning  of 
coal! 


CHAPTER   VIII. 

How  is  it  there  Is  any  radium  left? — The  parent  of  radium — Fixity 
of  ratio  between  the  quantity  of  uranium  and  radium  in  all  min- 
erals— Period  of  average  life  of  uranium — Relation  of  uranium  to 
radium — An  analogy  to  the  Glasgow  water  supply  system — Age  of 
pitchblende — Radioactivity  of  uranium — Uranium  X — Uranium 
I  and  uranium  II — Uranium  not  the  direct  parent  of  radium — 
Growth  of  radium  by  uranium — Intermediate  transition-forms  of 
long  life — Ionium,  the  direct  parent  of  radium — The  stately  pro- 
cession of  elementary  evolution. 

ONE  of  our  chief  duties  will  be  to  follow  out 
this  theory  of  the  disintegration  of  atoms  in 
radioactivity.  The  bare  idea  of  elements  spon- 
taneously changing  raises  so  many  obvious  and 
apparently  insurmountable  difficulties  that  it  will  be 
interesting  to  consider  them  as  they  arise  and  to 
consider  what  answer  can  be  made  to  them.  To- 
night we  must  concentrate  on  one  of  the  chief  of 
these — a  difficulty  which  no  doubt  has  already  pre- 
sented itself  in  many  of  your  minds.  If  radium  is 
changing  at  the  rate  of  nearly  one  two-thousandth 
part  every  year,  how  is  it  that  there  is  any  radium 
left  at  the  present  time  ?  Even  at  the  beginning  of 
the  time  recorded  in  past  history  there  must  have 
existed  several  times  as  much  radium  as  there  is 
now,  if  the  rate  of  disintegration  has  been  constant 

1 68 


HOW  IS  JtADIUM  MAINTAINED?  l69 

over  that  period,  while  a  hundred  thousand  years 
ago  it  can  be  calculated  that  there  must  have  existed 
a  thousand  billion  times  as  much  as  to-day,  had  the 
steady  disintegration  been  going  on  at  its  present 
rate.  That  is  to  say,  even  if  the  whole  world  were 
originally  pure  radium,  in  a  period  of  time  brief 
compared  to  that  which  we  know  from  geological 
evidence  it  has  actually  been  in  existence,  there 
would  be  practically  none  left,  and  certainly  not  as 
much  as  actually  exists  to-day.  Or,  looking 
forward  instead  of  backward,  if  we  put  this  half- 
grain  of  radium  bromide  in  a  safe  place,  and  then 
could  revisit  the  earth  say  twenty-five  thousand 
years  hence,  we  should  find  less  than  one-thousandth 
part  of  it  remaining.  The  slow  disintegration 
would  have  done  its  work  and  changed  the  radium 
into  the  non-radioactive  elements  which  are  being 
formed  from  it.  This  question,  apparently  so 
insoluble,  in  reality  admits  of  the  most  direct  and 
satisfactory  answer  on  the  disintegration  theory  and 
serves  as  a  good  example  of  how  a  theory,  if  it  is 
worth  the  name,  must  be  able  to  predict  future  dis- 
covery as  well  as  to  explain  the  existing  facts. 

An  analogy  to  facts  we  have  already  discussed 
will  help  us  to  find  the  solution  of  this  difficulty. 
In  the  emanation  of  radium  we  have  become 
acquainted  with  a  body  changing  so  rapidly  that  at 
the  end  of  a  month  none  of  the  original  quantity 


i?o  HOW  IS  XADIUM  MAINTAINED? 

remains.  How  is  it  there  is  any  emanation  in 
existence  at  all  ?  Because  it  is  being  reproduced  as 
fast  as  it  disappears.  Is  there  any  reproduction  of 
radium  going  on,  balancing  the  effect  of  its  disin- 
tegration and  maintaining  its  quantity  from  age  to 
age  ?  Radium  is  the  direct  parent  of  the  emana- 
tion. Itself  changing  more  than  a  hundred  thousand 
times  slower  than  its  product,  it  maintains  the  quan- 
tity of  emanation  in  existence  over  a  period  a  hun- 
dred thousand  times  longer  than  would  otherwise 
be  the  case.  Is  there  then  a  parent  of  radium  ? 
Does  there  exist  any  other  element  producing 
radium  by  its  own  disintegration  as  fast  as  that 
already  in  existence  disappears  ? 

Do  not  regard  this  thirty  milligrams  of  radium 
bromide  as  something  merely  by  itself.  Consider 
its  history.  By  infinite  labour  and  patience  this 
tiny  quantity  of  radium  has  been  separated  from 
several  hundredweights  of  the  mineral  pitchblende. 
Suppose  in  this  operation  all  the  rest  of  the  mineral, 
after  the  extraction  of  the  radium,  were  preserved 
and  put  in  a  safe  place.  When  we  revisited  our 
specimen  of  radium  twenty-five  thousand  years 
hence,  and  found  practically  none  of  it  remaining, 
should  we  find  that  the  mineral  from  which  it  was 
extracted  had  in  the  meantime  grown  a  fresh  crop 
of  radium  ?  The  answer  is  that  we  should.  1 
This  was  one  of  the  first  predictions  made 


REPRODUCTION  OF  RADIUM  171 

from    the    theory  of    atomic     disintegration    and 
one    of    the     most    recent    to    be    confirmed    by 
experiment.     Long  before  the  data  were  available 
which   enabled   an   exact   estimate   of   the   life   of 
radium   to   be    calculated,   it  was   recognised   that 
radium,    though    at   first    sight   a   permanent   and 
primary  radio-element,  is  changing  so  rapidly  that, 
had  there  existed  no  process  in  which  fresh  radium 
is    supplied    to   replace  that  changing,   none  could 
possibly  have  survived    till    the  present  day,   and 
from   general    principles   it   was  possible  to  make 
a  shrewd  prediction  as  to  which  element  was  the 
parent  of  radium.     We  have  already  considered  the 
general  principles  which  enabled  the  prediction  that 
helium  was  one  of  the  ultimate  products  of  radio- 
active   changes    to    be    made.      Ultimate   products 
must    co-exist    with    the    radio-elements  producing 
them  in  all  the  natural  minerals  in  which  the  latter 
are    found.      Something    of    the    same    reasoning 
applies  to  the  parent  of  radium,  only  in  this  case  it 
is  far  more  definite  and  elegant.     The  parent  of 
radium  must  co-exist  with  radium  in  all  minerals  in 
which    radium    is    present.       Now    it   is   at   once 
obvious,  if  this  explanation  of  the  parent  of  radium 
is  to  meet  the   case,  that   such  a   body   must   be 
changing    very   much    more   slowly    than    radium, 
otherwise  there  would  arise  the  same  necessity  to 
assume   the  existence  of   a  parent  of   the  parent 


172  REPRODUCTION  OF  RADIUM 

as  there  is  of  a  parent  of  radium.  The  original 
first  parent  of  radium  must  be  changing  excessively 
slowly  to  maintain  a  steady  supply  of  radium  over 
long  epochs  of  geological  time. 

By  the  law  already  formulated  on  page  164,  in 
two  successive,   not  necessarily  consecutive,  disin- 
tegrations of  which  the  second  is  much  more  rapid 
than   the    first,   the   more    rapidly    changing   body 
accumulates    in    quantity    until    a   fixed    ratio   with 
respect  to   the  parent  body  is   attained,   and  this 
ratio  is  inversely  proportional  to  the  ratio  of  their 
respective  rates  of  change,  or  directly  proportional 
to  the  ratio  of  their  respective  periods  of  average 
life.     Let   us   apply   this   law.     The   parent    body 
is  the  parent  of  radium.     The  quantity  of  radium 
in    minerals    must    therefore   attain   a  fixed  ratio 
with    respect    to    the    quantity    of   the    parent    of 
radium,   and  this  ratio   is   the   ratio   of  the  period 
of  average  life  of  radium  to  that  of  its  parent.     The 
quantity  of  helium  that  accumulates   in   a  mineral 
continually  increases  as  time  goes  on,  assuming  the 
helium    does    not    succeed    in    escaping,    and    no 
definite  proportion  between  helium  and  radium  is 
to   be   expected.     But   the    case   is   different  with 
radium   and    its   parent.     There    must   be  a  fixed 
ratio,  independent   of  the  age   of  the  mineral  ex- 
amined.    As    the    original    first   parent    of  radium 


URANIUM  AND  RADIUM  173 

must  be  changing  excessively  slowly  to  survive 
geological  epochs  of  past  time,  there  must  be 
always  a  very  large  quantity  of  it  in  the  mineral. 
As  the  radium  is  changing,  from  the  standpoint 
of  geological  epochs  of  time,  very  rapidly,  there 
must  always  be  a  very  small  quantity  of  radium. 
Between  these  quantities  great  and  small  there 
must  exist  the  same  ratio  as  between  the  respective 
periods  of  average  life  of  the  two  bodies. 

A  very  cursory  examination  of  the  minerals  in 
which  Mme.  Curie  found  radium  was  sufficient  to 
point  strongly  to  the  probability  that  uranium  is 
the  primary  parent  of  radium.  Uranium  was,  as 
we  have  seen,  the  original  element  for  which  the 
property  of  radioactivity  was  discovered,  and  its 
radioactivity  is  several  million  times  more  feeble 
than  that  of  radium.  Now  the  radioactivity  depends 
only  on  the  atoms  actually  breaking  up,  and  there- 
fore in  comparing  uranium  with  radium  it  follows 
that  uranium  must  be  disintegrating  several  million 
times  more  slowly  even  than  radium,  so  that  if 
uranium  produces  radium  the  quantity  of  uranium 
must  be  several  million  times  greater  than  the 
quantity  of  radium  in  minerals.  But  this  is  exactly 
what  Mme.  Curie  found  to  be  the  case  in  the 
minerals  she  worked  up  for  radium.  So  that  from 
the  very  first  there  existed  a  strong  presumption 


174  URANIUM  AND  RADIUM 

that  uranium  is  the  original  parent  of  radium.     The 
evidence    in  support  of  this   view  at  the   present 
time  is  indirect,  but  quite  satisfactory.     We  owe 
it  to  the  careful  work  of  McCoy,  Strutt  and  Bolt- 
wood  that  the  genetic  relation  between  uranium  and 
radium   has   been   established.      They   determined 
the   ratio   between   the  quantities   of  uranium  and 
of  radium  in  a  large  number  of  minerals.     In  every 
mineral    examined    containing   uranium   there  was 
found  to  exist  a  direct  proportionality  between  the 
quantity    of   uranium    and    that    of    radium.      To 
Rutherford   and    Boltwood   together   we   owe   the 
exact  determinations  of  this  important  constant  of 
proportionality.     They   found   that   for  every  one 
part    of    radium    there     always    exists    3,200,000 
parts   of  uranium.      This    constant   gives  directly, 
unless    other    undetermined    factors    interfere,    the 
ratio  of  the  average  lives  of  the  two  elements.     As 
we  have  seen,  that  of  radium  is  2500  years.     Hence 
it  follows   that   that   of  uranium    is    8,000,000,000 
years.      Enormous    as    this    period    is,    it    is    not 
now   merely  a    deduced    or    calculated    value.       I 
obtained   the    same    result    by    direct    experiment 
from     the     rate    of    production    of    helium    from 
uranium. 

It  will  help  us   considerably   if   we   try   to   find 
some  analogy  to  the  important  and  intricate  rela- 


URANIUM  AND   RADIUM  175 

tions  that  exist  between  uranium  and  radium.  We 
may  take  for  illustration  the  magnificent  system  of 
waterworks  which  supply  this  city,  which  we  will 
suppose  have  been  given  over  to  us  by  the  Cor- 
poration to  control  for  the  purposes  of  our  illustra- 
tion. As  you  know,  we  in  Glasgow  are  supplied 
ultimately  from  Loch  Katrine  through  an  inter- 
mediate reservoir  at  Milngavie.  We  shall  first  cut 
off  Loch  Katrine  from  all  fresh  sources  of  supply 
of  water,  and  from  all  outlets  except  to  the 
intermediate  reservoir  at  Milngavie,  and  we  shall 
see  to  it  also  that  the  latter  receives  no  water 
except  from  Loch  Katrine,  and  delivers  none 
except  to  Glasgow.  We  shall  then  issue  to  our 
engineers  the  instructions  that  there  must  be 
delivered  every  hour  at  Milngavie  from  Loch 
Katrine  approximately  one  eight-millionth  part 
of  the  total  store  of  water  in  Loch  Katrine, 
and  from  Milngavie  to  Glasgow  every  hour  one 
two -thousand -five -hundredth  part  of  the  total 
store  of  water  at  Milngavie.  Then,  if  instead 
of  hours  we  read  years,  the  quantity  of  water 
in  Loch  Katrine  represents  the  quantity  of 
uranium,  and  the  quantity  of  water  in  Milngavie 
that  of  radium.  For  the  sake  of  brevity  we  shall 
term  Loch  Katrine  the  source  and  Milngavie  the 
reservoir. 

First  we  shall  suppose  that  our  regulations  have 


1 76  URANIUM  AND  RADIUM 

been  in  operation  already  a  considerable  number  of 
hours,  as  this  is  the  condition  in  which,  reading  years 
for  hours,  we  find  uranium  and  radium  together  in 
minerals  in  Nature,  for  example,  in  a  piece  of 
pitchblende.  What  rejation  will  the  quantity  of 
water  in  the  source  bear  to  the  quantity  in  the 
reservoir,  that  is,  the  quantity  of  uranium  to  the 
quantity  of  radium  ?  The  amount  of  water  the 
reservoir  receives  is  quite  independent  of  the  amount 
it  contains,  but  the  amount  it  delivers  is  propor- 
tional to  the  amount  it  contains.  Similarly  the 
amount  of  radium  produced  from  uranium  does  not 
depend  at  all  on  the  amount  of  radium  already 
present,  while  the  amount  that  itself  changes  de- 
pends only  on  and  is  proportional  to  the  amount 
present.  Nevertheless,  we  shall  find  that  there 
is  about  three  million  times  more  water  in 
the  source  than  in  the  reservoir.  Because  only 
under  this  condition  is  the  intake  of  the  reservoir 
equal  to  the  outflow  from  the  reservoir,  that  is,  the 
production  of  new  radium  equal  to  the  disappear- 
ance of  the  old.  Imagine,  for  example,  that  there 
was  just  twice  as  much  water  as  this  ratio  in  the 
reservoir,  then  twice  as  much  would  flow  out  as 
flows  in,  and  the  supply  in  the  reservoir  would  be 
rapidly  depleted.  Or,  if  there  were  but  one  half 
as  much  in  the  reservoir,  twice  as  much  would  flow 
in  as  out,  and  the  supply  in  the  reservoir  would 


AGE    OF  PITCHBLENDE  *77 

increase.  In  either  case,  intake  and  outflow  would 
ultimately  become  equal,  and  no  further  change 
would  then  occur  until  both  the  source  and  the 
reservoir  were  empty.  But  let  us  now  disconnect 
Loch  Katrine  from  Milngavie  reservoir,  which  is 
equivalent  to  separating,  as  Mme.  Curie  did,  the 
radium  from  the  uranium  in  pitchblende.  Obviously 
the  reservoir  by  itself  will  now  be  able  to  supply 
water  for  a  very  much  shorter  time  than  it  did 
before,  and,  in  general,  with  the  conditions  stated, 
source  and  reservoir  together  will  last  three  million 
times  longer  than  the  source  alone.  The  radium 
on  the  table  will  have  half  disintegrated,  so  that 
only  half  will  remain,  in  about  1750  years.  Where- 
as had  it  remained  in  the  mineral  associated 
with  its  parent  uranium,  the  quantity  of  radium 
in  the  mineral  will  not  be  reduced  to  one  half 
what  it  is  now  until  5,000,000,000  years  have 
elapsed. 

Thus  we  can  say,  following  a  cautious  reservation 
once  made  by  Professor  Tait,  provided  the  causes 
that  are  now  at  work  have  always  been  in  con- 
tinuous operation  in  the  past  as  they  are  now,  and 
that  we  know  of  all  the  causes  that  have  been  at 
work,  5,000,000,000  years  ago  there  must  have 
been  about  twice  as  much  uranium  and  radium  in 
this  piece  of  pitchblende  as  there  is  to-night. 
Since,  however,  there  is  actually  in  this  pitchblende 


N 


178  MAINTENANCE   OF  RADIUM 

now  over  50  per  cent  of  uranium,  it  is  not  possible 
that  it  can  have  been  in  existence  in  its  present 
form  more  than  5,000,000,000  years.  But,  even 
from  a  geological  point  of  view,  this  is  a  very  long 
period  of  time  indeed ;  longer,  perhaps,  than  it 
would  be  profitable  in  the  present  state  of  science 
to  push  back  our  inquiries.  That,  then,  is  the 
position  with  regard  to  the  maintenance  of  radium 
in  Nature.  Even  when  we  deliberately  leave  out 
of  account  the  possibility  there  may  exist  in  Nature 
entirely  unknown  processes  replenishing  the  sup- 
plies of  uranium,  just  as  there  are  replenishing 
Loch  Katrine,  there  is  no  difficulty  in  accounting 
for  the  continuous  maintenance  of  radium  over  a 
period  of  the  past  as  great  as,  or  greater  than, 
there  is  any  reason  to  believe  the  earth  has  been 
in  existence  in  its  present  condition.  This  is  as 
far  as  we  need  pursue  our  analogy  for  the  moment, 
but  we  shall  again  find  it  useful  at  a  later  period. 
We  must  pass  on  to  another  aspect  of  the 
question. 

At  this  stage  it  will  be  well  to  make  a  short  digres- 
sion into  the  radioactivity  of  uranium  itself,  and  how 
it  is  explained  on  the  theory  of  atomic  disintegra- 
tion. Uranium  and  its  compounds  in  their  normal 
state  give  out  both  a-  and  /3-rays.  As  in  all  other 
cases,  the  /3-rays,  being  photographically  the  most 


URANIUM  X  179 

active  and  being  the  more  penetrating,  were  the 
first  chiefly  studied.  Sir  William  Crookes  and  also 
M.  Becquerel  found  that  by  certain  chemical  pro- 
cesses a  new  substance  in  minute  quantity  could  be 
separated  from  uranium,  to  which  Crookes  gave  the 
name  Uranium  X,  and  this  new  body  produced 
the  whole  of  the  photographic  activity  of  uranium. 
The  uranium  after  this  treatment  no  longer  affected 
a  photographic  plate.  Crookes  concluded  that  the 
radioactivity  was  due  in  reality  to  the  presence  of 
the  foreign  substance  in  minute  amount,  which  he 
called  uranium  X,  and  that  pure  uranium  was  not 
radioactive.  I  repeated  these  experiments,  and 
found  that  only  the  /3-rays  of  uranium  belonged 
to  the  uranium  X.  Uranium  freed  from  uranium  X 
gave  its  normal  amount  of  a-rays.  Then  it  was 
found  that  the  /3-radiation  of  uranium  X  decayed 
steadily  in  a  geometrical  progression  with  the  time, 
whereas  the  uranium  that  had  been  freed  from 
uranium  X  and  at  first  gave  no  /3-rays,  gradually 
and  completely  recovered  its  power  of  producing 
/3-rays.  Uranium  grows  uranium  X,  in  exactly  the 
same  way  as  radium  grows  the  emanation.  The 
activity  of  uranium  X  after  separation  from  uranium, 
consisting  entirely  of  /3-rays,  steadily  decays  in  a 
geometrical  progression  with  the  time,  falling  to 
one  half  the  initial  value  in  24 '6  days.  The  average 
life  of  uranium  X  is  thus  35-5  days.  A  variety  of 


i8o  URANIUM  I  AND  II 

evidence,  some  of  which  may  be  dealt  with  more 
profitably  later,  has  lately  established  the  conclusion 
that  the  change  suffered  by  the  uranium  atoms  when 
the  a-particles  are  expelled,  is  not,  as  first  supposed, 
a  single  change.  The  substance  uranium,  which 
chemists  have  hitherto  considered  an  element,  differs 
from  every  other  known  substance  expelling  a-rays, 
in  that,  per  atom  disintegrating,  two  ^-particles  are 
expelled  instead  of  one.  Moreover,  these  two 
a-particles  are  expelled  at  slightly  different  initial 
velocities,  with  the  result  that  the  "  ranges  "  of  the 
two  sets  of  a-rays  in  air  are  slightly  different.  Most 
probably  the  two  a-particles  are  not  expelled  from 
the  uranium  atom  simultaneously  but  successively. 
In  consequence,  what  chemists  hitherto  have 
accepted  as  a  single  element  is,  in  reality,  a  mixture 
of  two,  chemically  so  much  alike  that  they  have  not 
yet  been  separated,  the  first  having  the  atomic  weight 
238*5,  and  which  has  been  termed  provisionally 
uranium  I  ;  the  second,  resulting  after  the  expulsion 
of  the  first  a-particle,  having  the  atomic  weight 
234*5.  It  has  been  termed  uranium  II.  It  is 
probable  that  this  uranium  II  is  present  in 
relatively  very  insignificant  proportion  by  weight, 
although  it  contributes  one-half  of  the  total  «-radia- 
tion.  Its  period  of  life  can  only  be  estimated  from 
very  indirect  and  incomplete  data  at  the  present 
time,  but  this  estimate,  such  as  it  is,  attributes  a 


URANIUM  X  181 

period  to  the  substance  of  about  two  million  years. 
We  must  leave  for  the  present  these  highly  inter- 
esting recent  developments  to  resume  the  considera- 
tion of  uranium  X.  The  disintegration  of  uranium 
up  to  the  point  so  far  discussed  is  represented  on 
the  following  scheme,  though  it  may  again  be  em- 
phasised that  the  evidence  as  regards  the  existence 
and  properties  of  uranium  II  is  still  very  indirect. 


Uranium  Uranium  Uranium 

I.                   II.  X. 

8,000, oco,ooo  2,000,000  35 '5 

years.  years  (?).  days. 

FIG.  25. 

This  is  as  far  as  the  methods  of  radioactivity 
enable  us  to  trace  the  disintegration  of  uranium 
at  the  present  time.  The  substance  produced — 
uranium  X — is  only  an  ephemeral  transition-form, 
lasting  on  the  average  35*5  days,  and  when  it  dis- 
integrates, the  process  appears  to  come  to  a  stop 
so  far  as  our  experimental  methods  have  yet  been 
able  to  disclose. 

Now,  on  the  view  that  has  been  developed  that 
uranium  is  the  parent  of  radium,  it  is  natural  to 
suppose  that  uranium  X  in  the  course  of  time 
turns  into  radium.  A  little  consideration  will  show 


182  ATTTEMPS  TO  DETECT 

that  if  this  were  the  case  it  might  easily  be 
overlooked  at  first  on  account  of  the  very  long 
period  of  life  of  radium  compared  with  that  of 
uranium  X.  As  already  explained  (p.  127),  chemi- 
cal and  spectroscopic  methods  of  detecting  matter 
depend  only  on  quantity,  but  radioactive  methods 
depend  upon  quantity  divided  by  life.  Assuming 
equal  effects  produced  in  the  disintegration  of  an 
atom  of  uranium  X  and  of  an  atom  of  radium, 
since  the  life  of  the  latter  is  30,000  times  that  of 
the  former,  it  will  be  necessary  to  have  30,000  times 
as  much  radium  as  of  uranium  X  to  produce  equal 
radioactive  effects. 

In  1903  I  started  a  series  of  special  experiments 
which  have  been  continued  ever  since  I  came  to 
Glasgow,  partly  in  conjunction  with  Mr.  T.  D. 
Mackenzie,  to  see  whether  uranium  does,  in  fact, 
produce  radium.  The  uranium,  after  being  purified 
as  completely  as  possible  by  chemical  methods  from 
radium,  is  left  sealed  up  in  a  flask  and  is  periodic- 
ally tested  to  see  if  a  growth  of  radium  has 
occurred.  The  method  of  testing  for  minute  traces 
of  radium  is  a  very  simple  and  accurate  one,  allow- 
ing quantities  of  radium  of  only  a  few  million- 
millionths  part  of  a  grain  to  be  detected  with 
certainty  and  measured  with  exactitude.  Use  is 
made  of  the  characteristic  emanation  generated 


A   GROWTH  OF  RADIUM  183 

by  radium.  Uranium  does  not  generate  any  emana- 
tion. The  uranium  solution  to  be  tested  for 
radium,  after  standing  sealed  up  in  a  glass  flask  for 
a  period  of  at  least  a  month  to  allow  the  equi- 
librium quantity  of  emanation  to  accumulate,  is 
boiled  in  a  vacuum,  and  the  gases  expelled  are 
collected  and  introduced  into  a  sensitive  gold-leaf 
electroscope.  If  radium  is  present  in  the  solution, 
its  emanation  causes  the  leaf  to  lose  its  charge, 
and  the  rate  at  which  the  discharge  occurs  under 
defined  conditions  can  be  used  accurately  as  a 
measure  of  the  amount  of  radium  present.  The 
test  is  qualitative  as  well  as  quantitative,  and  there 
is  no  possibility  of  making  a  mistake  as  to  the 
identity  of  the  emanation  and  of  the  radium  from 
which  it  is  formed. 

The  present  result  of  these  experiments,  while 
they  furnished  the  first  evidence  of  a  growth  of 
radium,  withal  in  very  minute  amount,  have  clearly 
proved  that  this  growth  is  not  due  to  uranium. 
In  the  first  experiments  the  uranium  salt  was  only 
specially  purified  from  radium,  not  from  any  other 
impurities  that  might  have  been  present,  derived 
from  the  minerals  from  which  uranium  is  obtained, 
and  a  very  slow  growth  of  radium  from  the  prepara- 
tion was  actually  observed. 

In  later  experiments  more  perfect  methods  of 
purifying  the  uranium  initially  were  adopted,  with 


184  INTERMEDIATE  PRODUCTS 

the  result  that  the  growth  now  of  radium  occurred 
chiefly  in  the  impurities  separated,  whilst  the  growth 
in  the  purified  radium  was  reduced  to  an  excessively 
minute  amount.  In  these  the  greatest  growth 
recorded  is  only  one  fifty-millionth  of  a  milligram  of 
radium  after  six  years.  At  this  rate,  even  at  the 
present  enormous  price  of  radium,  it  would  require 
sixty  thousand  years  to  produce  one  pennyworth. 

Now  if  uranium  X,  when  it  disintegrates,  pro- 
duced radium  directly,  then  with  the  quantities  of 
materials    used    in    these    later    experiments,    the 
amount  formed  in  a  single  hour  would  be  greater 
than  has  actually  been   formed    in   six   years.     In 
the  earlier  experiments,  with  not  specially  purified 
uranium,    the   growth    of    radium,    although   quite 
detectable,  was  still  only  one  thousandth  part  of 
what  would  have  occurred  had  uranium  X  changed 
directly  into  radium.      In  spite  of  this  apparently 
conclusive  negative  result,   it  is  practically  certain 
at  the  present  time   that  uranium   is   the   original 
parent  of  radium,  and  that  in  the  course  of  years 
our  preparations  will  begin  to  grow  radium. 

The  natural  explanation  of  this  failure  to  detect 
a  growth  of  radium  from  uranium  is,  that  one  or 
more  intermediate  bodies  of  long  life  exist  in  the 
disintegration  series  between  uranium  and  radium. 
On  the  analogy  proposed,  this  means  that  between 


INTERMEDIATE  PRODUCTS  185 

Loch  Katrine  and  Milngavie  reservoir  one  or 
more  large  intermediate  reservoirs  exist,  which 
have  to  fill  up  before  the  water  reaches  Milngavie. 
Uranium  X  represents  the  first  of  such  a  series  of 
intermediate  reservoirs,  it  is  true,  but  owing  to  its 
short  period  of  life  and  the  large  fraction  of  the 
total  quantity  always  passing  through  on  the  rray 
to  the  next,  such  a  reservoir  would  be  an  extremely 
small  one,  and  for  periods  such  as  we  are  considering 
its  effect  on  the  flow  would  be  practically  negligible. 
It  would  be  quite  otherwise  if  one  or  more 
reservoirs  as  large  as  Milngavie — if  one  or  more 
intermediate  substances  as  long-lived  as  radium- 
existed  in  the  series.  I  well  remember  one  fact 
told  me  by  the  engineer  in  charge  of  the  mag- 
nificent scheme  of  waterworks,  supplying  the  mines 
at  Kalgurli,  in  Western  Australia,  from  a  source 
near  the  coast  across  three  hundred  miles  of  desert. 
There  are  several  intermediate  reservoirs  on  the 
way.  The  plant  installed  is  capable  of  pumping 
five  million  gallons  of  water  daily,  and  yet  it  took 
a  period  of  many  weeks  since  pumping  operations 
began  before  the  water  appeared  in  Kalgurli. 
When  uranium  is  carefully  purified  from  all  other 
substances  one  can  be  sure  that  one  starts  with  all 
the  intermediate  reservoirs  empty,  that  is,  with  none 
of  the  intermediate  substances  present.  Water  is 
flowing  steadily  from  the  source  all  the  time,  as  the 


1 86  INTERMEDIATE  PRODUCTS 

disintegration  of  uranium  is  always  going  on.  We 
have  been  watching  and  waiting  seven  years  at  the 
radium  reservoir — strictly  speaking,  at  the  one 
beyond  radium,  since  the  emanation  of  radium,  not 
radium  itself,  is  actually  employed  for  the  test. 
But  the  flow  has  not  reached  there  yet  and  the 
radium  reservoir  remains  practically  as  empty  as  at 
the  start.  But  there  is  no  doubt  it  will  come,  and 
there  is  good  reason  to  expect  that  some  of  us,  at 
least,  will  be  still  alive  when  it  arrives. 

It  is  not  beyond  the  resources  of  mathematics  to 
find  out  a  good  deal  about  these  intermediate 
reservoirs.  The  present  results  indicate  that  if 
there  is  but  one  long-lived  intermediate  body 
between  uranium  and  radium,  then  its  period  of 
average  life  must  be  at  least  100,000  years,  that  is, 
forty  times  that  of  radium  itself.  On  our  analogy, 
then,  between  Loch  Katrine  and  Milngavie,  there 
must  exist  a  reservoir  of  forty  times  the  capacity 
of  Milngavie,  provided  there  is  only  one.  There 
may  be  several.  Several  small  ones  would  be 
much  more  effective  than  one  large  one.  Since 
the  equilibrium  quantity  to  which  an  intermediate 
body  accumulates  is  proportional  to  its  period  of 
average  life,  then  if  there  is  only  one  intermediate 
parent  of  radium  between  radium  and  uranium,  there 
must  be  forty  times  as  much  of  it  in  minerals  con- 
taining radium  as  there  is  of  radium  itself. 


THE  DIRECT  PARENT  OF  RADIUM         187 

This  leads  me  to  the  next  step.  The  failure 
to  detect  a  production  of  radium  from  uranium 
merely  foreshadowed  the  discovery  of  one  or 
more  intermediate  substances  of  long  period  of 
life.  Boltwood  in  America  has,  however,  actually 
succeeded  in  isolating  one  from  minerals  contain- 
ing radium,  and  this  one  proves  to  be  the 
direct  parent  of  radium.  Whether  there  are 
others  it  remains  to  be  seen.  The  one  found 
possesses  the  property  of  producing  radium  directly 
from  itself  by  disintegration,  and  it  has  been 
called  Ionium.  It  expels  a-rays  during  its  dis- 
integration into  radium,  and  these  a-rays  possess 
a  relatively  low  velocity.  Their  range  is  very 
little  more  than  one  inch  of  air.  Chemically, 
ionium  resembles  thorium  so  completely  that  the 
two  substances,  if  mixed,  cannot  be  separated. 
This  gives  the  means  of  separating  the  new 
body  from  minerals.  Some  thorium  is  added 
and  separated  by  the  well-known  methods  of 
chemical  analysis.  It  is  then  purified  as  com- 
pletely as  possible.  The  parent  of  radium  is  not 
separated  from  the  thorium  by  this  treatment, 
although  all  other  substances  are.  The  chemical 
resemblance  between  the  two  substances  is 
analogous  to  that  between  radium  and  barium  or 
polonium  and  bismuth,  and  does  not  signify  any 
genetic  connection  between  the  substances. 


S  TffE  PRODUCT  OF  URANIUM 

The  disintegration  series  thus  reads : — 


Uranium  L  Uranium  IL  Uranium  JL  Ionium  Radium.     Emanation. 

;,ooa*OQQ»ooo  2,000,000      3^$  days.  100,000         2,500        $^6  *is. 

yeais.  -  yean  (?)          y«urs. 

FIG.  t& 

as  far  as  we  have  yet  considered  it.     In  the  centre 
is  placed  the  known  or  presumed  atomic  weights  of 
the  various  bodies.     It  is  to  be  noticed  that  a  gap 
still  remains  between  uranium  X  and  the  parent  of 
radium.     As  the  mass  of  the  ^-particle  is  practically 
negligible,  the  product  of  uranium  X  should  still 
have  an  atomic  weight  of  230,  so  that  there  are 
presumably  no  more  changes  in  which  a-particles 
are  expelled.      There  is  little  doubt  that  the  gap 
will  soon  be  filled  up.     All  that  it  is  required  now  is 
to  prove  that  uranium  X  is  the  parent  of  ionium. 
A  long  research,  which  involved  the  repeated  separa- 
tion of  the  uranium  X  from  a  hundredweight  of 
pure  uranium  nitrate,  failed  to  establish  this,  doubt- 
less  on  account  of  the  very  long  period  of  ionium. 
Indeed,  it  is  possible  to  say  now  that  to  command 
success,  and  settle  the  question  definitely,  half  a  ton 
of   uranium   nitrate  would   probably   be   required. 
The  reason  for  this  is,  as  already  explained,  the 
enormous  period  of  ionium  by  comparison  with  that 
of  uranium  X»  which  an  indirect  estimate,  to  be 
later  considered,  indicates  to  be  about  200,000  years. 


ELElfENTAR  Y  E  VOL  VT2ON  1 89 

So    far,    then,    as    we    have    inquired,    uranium, 
uranium  X,  the  parent  of  radium,  radium,  and  the 
emanation  represent  respectively  the  starting-point 
and  the  four  successive  stopping-stations  in  the  long 
journey  of  continuous  devolution  from  the  heaviest 
and  most  complex  atom  known  into  less  heavy  and 
complex  atoms  which  is  going  on  around  us,  or,  to 
preserve  our  original  analogy,  the  source  and  four 
successive    intermediate    reservoirs  in   the    flow  of 
elementary  evolution.      "  All  things  flow  "  was  one 
of  the  dogmas  of  ancient  philosophy,  and  in  this,  as 
in  many  others,  the  ancients  guessed  truer  than  they 
knew.      Instead  of  four  stopping-stations  or  inter- 
mediate   reservoirs    in    this   stately   procession    of 
elements     disclosed     by    radioactivity,    there    are 
already  known  no  less  than  eleven,  starting  from  the 
element  uranium,   but  for  our  present  purposes  of 
illustration    these    four  will   suffice.      But   this   new 
transformation  scene   on  which  the  curtain  of  the 
twentieth  century  has  been  rung  up,  beginning  as  it 
has    done    with    the    transformation    of    the    most 
fundamental    and    permanent    of    the     existences 
which  physical  science  has  recognised  in  the  past, 
extends  beyond    physical  science  and  transfigures 
with  new  light  some  of  the  most  fundamental  and 
permanent  ideas  which  in  one  form  or  another  are 
deep-rcoted  in  the  world's  philosophies. 


CHAPTER   IX. 

The  subsequent  changes  of  radium — The  induced  or  excited  radio- 
activity— The  active  deposit  of  radium — The  disintegration  of  the 
emanation — Radium  A,  B,  C — Experiments  with  the  active  deposit 
— Radium  A  gives  only  a-rays  and  has  a  very  short  life — Radium  B 
gives  no  rays — Radium  C  gives  a-,  /3-,  and  -y-rays — The  emanation 
only  gives  a-rays — The  later  slow  changes  of  radium — Radium 
D,  E,  and  F — Polonium — Its  identity  with  radium  F — The  last 
disintegration— What  is  the  ultimate  product  ? 

WE  have  attempted  to  trace  radium  to  its 
source.  It  remains  to  follow  through  its 
disintegration  briefly  to  the  end.  This  was  a  task 
to  which  Rutherford  particularly  devoted  himself, 
after  the  main  principles  of  atomic  disintegration 
had  become  familiar,  with  the  consequence  that, 
with  the  exception  of  a  lacuna  here  and  there  still 
to  be  supplied,  our  knowledge  of  the  whole  process 
from  the  start  to  finish  is  now  tolerably  complete. 
In  addition,  some  new  considerations  have  trans- 
pired which  concern  us  nearly  in  the  broad  general 
application  of  the  principles  of  atomic  disintegration, 
so  that  for  this  reason,  if  for  no  other,  the  work 
claims  our  attention. 

Most  of  you  who  have  read  at  all  in  the  subject 

will  be  aware  of  one  mysterious  and  extraordinary 

190 


SUBSEQUENT  CHANGES  OF  RADIUM       191 

power  possessed  by  radium,  which  I  have  hitherto 
carefully  avoided  all  mention  of,  not  wanting  to 
have  too  many  irons  in  the  fire  at  once.  Radium 
possesses  the  power  of  endowing  with  some  of  its 
own  radioactivity  neighbouring  objects.  Thorium, 
which  is  very  like  radium  in  many  ways,  particularly 
in  giving  a  gaseous  emanation  (which,  however, 
has  the  very  short  period  of  average  life  of  only 
a  little  over  a  minute),  also  possesses  a  similar 
power.  The  phenomenon  was  discovered  by  the 
Curies  for  radium  and  termed  "induced  radio- 
activity," and  for  thorium  simultaneously  by 
Rutherford  and  termed  "  excited  radioactivity." 
With  the  explanation  of  the  property  the  original 
names  have  largely  fallen  into  disuse.  We  shall,  as 
usual,  confine  ourselves  to  the  case  of  radium.  Any 
object  left  in  the  immediate  neighbourhood  of  a 
radium  salt  becomes  radioactive,  but  after  it  is 
removed  the  radioactivity  decays  away  rapidly  and 
almost  completely,  abnormally  at  first,  but  subse- 
quently in  a  geometrical  progression  with  the  time, 
with  a  half-value  period  of  about  thirty  minutes. 
The  temporary  activity  so  "  induced  "  consists  of 
a-,  /3-,  and  y-  rays.  The  activity  exists  as  an  invisible 
film  or  deposit  over  the  surface  of  the  object 
rendered  radioactive,  for,  by  sand-papering,  the 
activity  can  be  rubbed  off  and  then  is  found  on 
the  sand-paper.  It  is  now  customary  in  con- 


iQ2  ACTIVE  DEPOSIT  OF  RADIUM 

sequence  to  refer  to  it  as  the   "  active  deposit  of 
radium." 

This  power  is,  strictly  speaking,  not  a  property  of 
radium  itself,  for  if  the  radium  is  contained  in  a 
completely  closed  vessel — it  does  not  matter  how 
thin-walled  so  long  as  it  is  air-tight — no  radio- 
activity whatever  is  produced  outside.  The  first 
step  in  understanding  the  nature  of  the  phenomenon 
consisted  in  tracing  it  to  the  action  of  the  emanation 

o 

of  radium.  In  the  ordinary  condition  the  emanation 
is  always  diffusing  away  to  some  extent  from  radium 
salts  unless  they  are  contained  in  air-tight  vessels. 
The  " active  deposit"  is  the  product  of  the  disin- 
tegration of  the  emanation.  Just  as  radium  cannot 
exist  without  continuously  producing  the  emanation, 
so  in  turn  the  emanation  cannot  exist  without  con- 
tinuously producing  this  active  deposit.  In  any 
vessel  containing  radium  emanation  this  body  is 
being  continuously  deposited  on  the  walls  of  the 
vessel,  so  that  if  the  emanation  is  at  any  time 
blown  out,  the  active  deposit  remains  behind. 
Radium  expels  one  a-particle  and  changes  into  the 
emanation.  The  emanation  expels  a  second  a-par- 
ticle and  changes  back  again  into  a  solid,  or  at  least 
into  a  non -gaseous  form  of  matter,  the  first  of  the 
"  active  deposit  "  group.  The  latter  in  turn  expels 
more  «-  and  also  /S-particles,  and  so  the  course  of 
successive  disintegrations  goes  on.  In  the  active 
deposit  itself  at  least  three  changes  follow  one 


ACTIVE  DEPOSIT  OF  RADIUM  193 

another   with    great    rapidity,   so    that   the   analysis 
of  them  proved  a  complicated  task. 

You  know  that  if  a  moisture-laden  atmosphere  is 
sufficiently  chilled,  the  vapour  of  water  condenses 
directly  into  the  solid  form,  and  a  snowstorm 
results.  Something  of  this  kind  is  always  happen- 
ing in  an  atmosphere  containing  the  radium 
emanation.  Every  second  two  out  of  every  million 
of  the  atoms  of  emanation  disintegrate,  expelling 
a-particles  and  leaving  a  solid  residue,  so  that  there 
is  a  sort  of  continuous  snowstorm  silently  going  on 
covering  every  available  surface  with  this  invisible, 
unweighable,  but  intensely  radioactive  deposit. 
Unlike  snow,  however,  the  particles  of  this  active 
deposit  are  charged  with  positive  electricity,  so  that 
if  two  surfaces  are  provided,  one  charged  nega- 
tively and  the  other  positively,  the  deposit  is 
attracted  almost  entirely  to  the  negatively  charged 
surface.  The  other  surface  repels  the  particles  and 
so  does  not  get  coated.  By  making  the  negatively 
charged  surface  very  small  the  active  deposit  can 
be  almost  entirely  concentrated  upon  it.  This 
enables  me  to  show  you  more  effectively  the  pro- 
duction of  the  active  deposit  from  the  emanation 
and  some  of  its  chief  properties.  The  separation 
of  the  non-volatile  product  of  a  volatile  parent  or 
emanation  by  this  use  of  a  negatively  charged  sur- 
face is  a  very  simple  operation,  much  more  so  than 


i94  RADIUM  A,  B,  AND  C 

when  the  parent  substance  is  non-volatile  and  the 
recoil  of  the  product  is  used  to  effect  its  separation 
and  concentration  on  a  negatively  charged  surface, 
as  discussed  on  p.  144. 

It  would  take  us  too  long  and  too  far  if  we 
attempted  first  to  study  these  properties,  and  then 
tried  from  them  to  deduce  their  explanation.  It 
must  suffice  if  I  give  you  first  the  explanation  of 
the  facts  according  to  the  theory  of  atomic  dis- 
integration and  then  illustrate  as  many  of  the 
points  in  it  as  possible  experimentally.  I  have  said 
that  after  the  disintegration  of  the  emanation  at 
least  three  successive  disintegrations,  following  one 
another  rapidly,  occur.  The  bodies  produced  are 
referred  to  as  Radium  A,  Radium  B,  Radium  C, 
in  order  to  avoid  the  necessity  of  inventing  a  host 
of  new  names  for  bodies  having  such  fleeting 
existence  (Fig.  27). 


Radium.     Emanation.  Radium  A.  Radium  B.  Radium  C. 


Active  deposit  of  rapid  change. 

2,500  years.     5-6  days.  4-3  38-5  28'! 

minutes.       minutes.        minutes. 

FIG.  27. 

As  before,  the  presumed  atomic  weights  are 
placed  inside  the  circles  corresponding  with  the 
successive  products.  The  periods  of  average  life 


RADIUM  A,  B,  AND   C  195 

are  placed  below.  The  symbol  08)  here  and 
throughout  indicates  that  #-rays  are  expelled,  but 
that  they  are  not  the  normal  penetrating  /3-rays,  but 
rays  akin  to  the  cathode-rays  in  their  low  pene- 
trating power  and  low  velocity.  They  only  come 
into  evidence  in  special  experiments,  and  are  not  of 
great  general  importance.  The  first  body  produced 
from  the  emanation,  radium  A,  changes  with  great 
rapidity  with  a  period  of  average  life  of  4*3  min- 
utes, expelling  an  a-particle.  The  body  radium  B 
resulting  undergoes  a  change  which  was  at  first 
thought  to  be  entirely  "rayless."  Neither  a-  nor 
£-rays  of  the  ordinary  kind  can  be  detected, 
although  a  very  feebly  penetrating  /3-ray  is  produced, 
which  we  need  not  further  consider.  The  period 
of  this  substance  is  38*5  minutes.  The  body  pro- 
duced, radium  C,  changes,  expelling  both  a-  and 
/3-particles  and  7-rays  also.  The  period  is  28*1 
minutes.  It  is  probable  that  this  change  is  complex, 
and  that  the  /3-  and  7-rays  are  given  off  in  a  separate 
change  to  that  in  which  the  a-rays  result.  The 
point  is  still  being  investigated,  and  need  not  further 
concern  us  now. 

We  started  our  description  of  the  rays  of  radium 
with  the  statement  that  they  consisted  of  a-,  P-, 
and  7-rays.  One  of  the  most  interesting  points  of 
the  above  scheme  is  to  show  that  the  /3-  and  7-rays 
do  not  come  from  radium  itself,  any  more  than  they 


196  RADIUM  A,  B,  AND   C 

do  from  uranium  itself,  but  from  the  later  products. 
It  is  loose,  but  convenient,  to  talk  of  the  P-  and 
ry-rays  of  radium.  Really  we  mean  the  /9-  and 
7-rays  of  radium  C.  The  emanation,  like  radium 
itself,  gives  only  a-rays.  The  whole  of  the  /5-rays 
result  in  the  later  changes  of  the  active  deposit. 


FIG.  28. 

We  have  seen  that,  freshly  prepared  from  solution, 
radium  salts  give  only  a-rays.  The  £-  and  7-rays 
make  their  appearance  only  after  the  subsequent 
products  have  accumulated. 

On  the  table  there  is  a  small  glass  vessel  silvered 
internally  (Figs.  28  and  29)  containing  the  emana- 


FIG.  29.      Apparatus  for  obtaining  the  Active  Deposit  of  Radium. 


To  face  p.  196. 


EXPERIMENTS  WITH  THE  ACTIVE  DEPOSIT   197 

tion  from  half  a  grain  of  radium  bromide.  It  is 
arranged  so  that  steel  knitting-needles  can  be 
inserted  into  the  emanation  and  withdrawn  through 
a  glass  tube  held  in  a  cork.  The  needle  is  con- 
nected to  the  negative  pole  of  the  electric  supply 
and  the  silver  coating  to  the  positive  pole.  If  only 
the  point  of  the  needle  is  made  to  project  beyond 
the  glass  tube,  the  whole  of  the  active  deposit  can 
be  concentrated  on  the  point.  Some  hours  before 
this  lecture  a  needle — we  will  call  it  No.  i — was  so 
inserted,  and  by  now  its  point  should  be  coated 
to  its  maximum  degree  of  radioactivity  with  the 
products  of  the  disintegration  of  the  emanation. 
After  some  hours  the  products  all  arrive  at  the 
state  of  radioactive  equilibrium,  in  which  the 
quantity  is  at  its  maximum  for  all  the  products, 
radium  A,  radium  B,  and  radium  C,  as  much  of 
each  changing  as  is  produced  from  the  emanation. 
The  disintegrations  all  going  on  together,  the  wire 
should  give  a-,  /3-,  and  y-rays,  the  ft-  and  y-rays 
being  as  intense  as  those  given  from  the  half-grain 
of  radium  bromide  from  which  the  emanation  was 
derived.  Now  I  withdraw  No.  i  needle  from  the 
emanation,  and  with  the  room  darkened  we  will 
examine  its  active  deposit. 

To  detect  the  a-rays  we  will  use  a  glass 
translucent  screen,  thinly  coated  with  phos- 
phorescent zinc  sulphide  on  one  side.  I  bring  the 
point  of  the  needle  gradually  near  the  coated  side 


198   EXPERIMENTS  WITH  THE  ACTIVE  DEPOSIT 

of  the  screen.  As  soon  as  it  comes  within  a 
distance  of  three  inches  the  screen  lights  up,  and 
when  the  point  is  only  a  little  distance  removed 
from  the  screen  a  most  brilliant  phosphorescence 
is  produced.  Now  if  I  interpose  between  the  wire 
and  the  screen  a  single  sheet  of  paper,  the  effect 
practically  entirely  ceases.  The  a-radiations  pro- 
ducing this  effect  come  both  from  radium  A  and 
from  radium  C. 

To  detect  the  /3-rays  we  will  use  an  ordinary 
cardboard  X-ray  screen  of  barium  platinocyanide. 
Bringing  the  needle  behind  the  screen,  so  that  the 
rays  have  to  penetrate  the  cardboard,  you  observe 
the  screen  lights  up  as  brightly  as  with  half  a  grain 
of  radium  bromide  itself.  In  the  dark  I  happened 
actually  to  touch  the  back  of  the  screen  with  the 
active  needle-point,  and  in  so  doing  some  of  the 
active  deposit  has  been  transferred  to  the  back  of 
the  screen.  You  can  see  where  the  back  of  the 
screen  was  touched,  because  this  spot  still  glows 
though  the  needle  has  been  removed. 

If  now  the  needle  is  again  presented  to  the  back 
of  the  X-ray  screen  with  thin  pieces  of  metal  foil 
interposed,  you  see  that  the  rays  are  only  slightly 
stopped  by  having  to  traverse  the  foil.  When 
a  piece  of  thick  lead  sheet  is  interposed,  a  faint 
luminosity  on  the  screen  still  remains  produced  by 
the  y-rays.  In  fact  the  active  needle-point  gives 


EXPERIMENTS  WITH  THE  ACTIVE  DEPOSIT  199 

all  the  penetrating  rays  given  by  half  a  grain  of 
radium  bromide. 

It  is  now  several  minutes  since  the  needle  was 
removed  from  the  emanation.  If  we  now  again 
examine  the  a-rays  you  will  notice  they  already 
are  very  perceptibly  less  intense  than  at  first. 
Practically  all  the  radium  A,  of  which  the  period 
of  average  life  is  only  4*3  minutes,  has  already 
disintegrated,  and  in  consequence  the  a-rays  now 
come  only  from  the  radium  C,  and  are  only  half  as 
intense  as  at  first. 

Now  if,  instead  of  exposing  the  needle  to  the 
emanation  for  some  hours  so  as  to  allow  all  the 
successive  products  time  to  be  produced,  we  expose 
it  to  the  emanation  for  a  very  short  time,  say  for  five 
minutes  by  the  watch,  we  shall  get  quite  a  different 
set  of  effects.  Here  is  a  new  needle,  we  will  call 
it  No.  2.  Before  putting  it  in  I  will  test  it  with 
the  screen  to  show  you  that  at  present  it  is  an 
ordinary  needle,  not  at  all  radioactive.  We  will 
let  it  stay  in  the  emanation,  connected  to  the 
negative  pole  as  before,  for  five  minutes  and 
withdraw  it,  and  test  its  a-rays  immediately,  exactly 
as  before.  You  observe  that  it  is  already  giving 
a-rays  abundantly.  Comparing  it  with  No.  i,  the 
two  are  now  very  similar  in  their  a-ray-giving 
power,  No.  i  being  only  slightly  the  better.  The 
a-rays  from  No.  2  come  entirely  from  radium  A, 


200   EXPERIMENTS  WITH  THE  ACTIVE  DEPOSIT 

for  there  has  not  yet  been  time  for  any  appre- 
ciable quantity  of  radium  C  to  be  formed.  The 
a-rays  from  No.  i  come  entirely  from  radium  C, 
and  this  radiation  has  not  yet  had  time  appre- 
ciably to  decay.  Let  us,  however,  test  their  /3-rays. 
You  observe  that  No.  2  gives  no  /3-rays  worth 
considering,  whereas  No.  i  still  gives  /3-rays  in 
practically  undiminished  intensity.  Radium  A 
gives  no  /3-rays,  and  as  there  is  no  appreciable 
quantity  of  radium  C  formed  there  yet,  the  con- 
sequence is  that  No.  2  wire  gives  no  /3-rays. 

I  can  show  you  at  this  stage  a  very  striking 
experiment  with  another  needle,  No.  3,  which  has 
been  in  the  emanation  a  few  minutes.  I  take  it 
out  and  draw  the  point  once  through  a  piece  of 
emery  -  cloth  and  expose  the  latter  to  the  zinc 
sulphide  screen.  You  observe  that  a  single  rub 
has  removed  a  large  part  of  the  active  deposit 
from  the  needle  and  transferred  it  to  the  emery- 
cloth,  so  that  the  latter  makes  the  screen  glow 
almost  as  brilliantly  as  the  needles  themselves. 

Now  we  will  contrast  the  decay  of  the  activity 
of  the  needles  Nos.  i  and  2.  The  activity  due  to 
radium  A  by  itself  decays  very  rapidly,  half 
disappearing  every  three  minutes.  The  con- 
sequence is,  if  we  now  again  test  the  a-rays  of 
No.  2,  we  shall  find  they  have  already  nearly 


EXPERIMENTS  WITH  THE  ACTIVE  DEPOSIT  201 

disappeared,  whereas  No.  i  still  continues  to 
give  a-rays  at  about  the  same  strength  as  it  did 
when  last  examined.  In  ten  minutes  the  a-rays 
of  No.  2  practically  disappear. 

It  is  thus  not  difficult  to  give  you  a  certain 
amount  of  experimental  evidence  in  favour  of 
the  conclusion  that  the  first  change  of  the  active 
deposit  is  a  very  rapid  one  in  which  a-,  but 
no  /3-rays  are  expelled,  and  that  this  is  followed 
by  a  less  rapid  change  in  which  both  a-  and  /3-rays 
are  expelled.  It  is  more  difficult  to  give  you 
in  a  lecture  satisfactory  evidence  of  the  exist- 
ence of  radium  B,  a  body  not  itself  giving  rays, 
intermediate  between  the  first  and  second  changes 
in  which  rays  are  expelled.  If  we  examine  care- 
fully the  decay  of  the  a-  and  /3-rays  of  wire  No.  i, 
in  which  at  first  all  these  products  co-existed  in 
equilibrium,  we  shall  find,  as  already  shown,  that 
for  the  first  half-hour  after  removal  from  the 
emanation  the  /3-rays  suffer  hardly  any  change 
and  then  quite  definitely  the  decay  begins.  In 
the  next  half-hour  the  /3-rays  decay  to  one  half 
their  original  intensity,  and  the  decay  then  goes 
on  at  this  rate  regularly  and  continuously  to  the 
end.  After  two  hours  they  are  only  a  few  per 
cent  of  what  they  originally  were,  and  in  three 
or  four  hours  they  can  no  longer  be  detected. 
The  initial  pause  before  decay  begins  is  due  to 
the  quantity  of  radium  C  being  maintained,  in 


202         THE  RAYS  FROM  THE  EMANATION 

spite  of  the  fact  that  it  is  disintegrating  all  the 
time,  expelling  a-  and  /3-rays,  by  the  disintegra- 
tion of  radium  B.  The  latter  continues  to  supply 
new  radium  C  to  replace  that  disappearing  for 
the  first  half-hour  or  so  after  the  needle  is  re- 
moved from  the  emanation.  Exactly  the  same 
pause  occurs  in  the  decay  of  the  a-rays.  As  we 
saw  with  No.  i,  within  a  very  few  minutes  after  the 
needle  was  removed  from  the  emanation  the  a-rays 
had  decayed  very  perceptibly,  owing  to  the  dis- 
appearance of  the  a-ray-giving  radium  A.  Then, 
however,  no  further  change  occurred.  It  is  now 
about  half  an  hour  since  No.  i  was  first  tested, 
and  the  a-activity  is  about  the  same  as  it  was 
when  last  tested  twenty  minutes  ago.  The  a-rays 
of  No.  2  have  now  almost  completely  disappeared. 
If  we  continued  to  examine  No.  i,  we  should 
find,  from  now  on,  a  rapid  decay  of  both  a-  and 
/3-rays  at  the  same  rate,  so  that  at  the  end  of  the 
lecture  both  will  be  much  enfeebled,  and  by 
midnight  both  will  have  ceased  so  far  as  we 
could  tell  by  these  rough  methods. 

Now  that  we  have  finished  with  the  emanation 
used  in  the  preceding  experiments,  it  is  an 
interesting  experiment  to  show  that  itself  it  gives 
no  /3-r.ays.  If  we  blow  the  emanation  out  into 
a  U-tube  of  thin  glass  cooled  in  liquid  air,  it 


THE  LATER  SLOW  CHANGES  OF  RADIUM    203 

is  condensed  in  the  cold  tube.  The  tube  can 
then  be  sealed  up  to  prevent  the  emanation  from 
escaping.  The  tube  contains  some  phosphorescent 
zinc  sulphide  and  glows  brightly  owing  to  the 
a-rays  from  the  emanation  inside.  But  if  we  hold 
the  tube  against  the  X-ray  screen,  you  can  see 
that  no  penetrating  rays  come  from  the  tube.  The 
emanation  itself  gives  no  /3-rays,  only  a-rays.  By 
the  end  of  the  lecture,  however,  sufficient  radium  C 
will  probably  have  been  formed  inside  the  tube 
to  give  an  appreciable  /3-radiation.  Owing  to 
the  existence  of  the  intermediate  body  radium  B, 
there  occurs  a  similar  pause  in  the  growth  of  /3-rays 
from  the  emanation  to  that  which,  as  we  have  seen, 
occurs  in  their  decay,  after  the  emanation  is  taken 
away.  But  in  two  or  three  hours  the  /3-rays  from  all 
the  needles  will  have  decayed,  and  that  from  the 
sealed  U-tube  will  have  reached  a  maximum. 

This  finishes  this  subject  and  brings  us  to  the 
next.  What  happens  to  radium  C  when  it  disinte- 
grates ?  Is  this  the  real  or  only  the  apparent  end 
of  the  process  ?  It  is,  in  fact,  a  very  long  way 
from  the  end.  Madame  Curie  discovered  that  the 
rapid  and  almost  complete  decay  of  the  active 
deposit,  at  the  end  of  a  few  hours  after  removal 
from  the  emanation,  is  not  in  fact  quite  complete. 
A  very  small  residual  radioactivity  remains  and  per- 


204  RADIUM  A   E,  AND  F 

sists  for  years.  The  series  of  changes  have  now 
entered  on  a  stage  which  is  as  slow  as  the  previous 
ones  were  rapid.  The  next  change  requires  almost 
as  many  years  as  the  last  required  minutes  for  com- 
pletion. The  effect  of  these  further  changes  is  in 
consequence  extremely  small,  but  they  last  a  very 
long  time.  Continuing  our  diagram  where  it  last 
ended  at  radium  C,  the  next  stage  is  represented  in 
Fig-  30- 


Radium  C.  Radium  D.    Radium  E.     Radium  F.  Radium  G. 

(Polonium.)     (Lead  ?) 

Active  deposit  of  slow  change. 
28'!  24  7'5  days.        202  days, 

minutes.        years  (?) 

FIG.  30. 

The  body  produced  from  radium  C,  radium  D,  has 
a  period  of  many  years.  It  is  too  early  yet  to  state 
it  exactly.  One  recent  estimate  makes  it  twenty- 
four  years.  No  very  important  rays  are  given  in  its 
change.  /3-rays,  however,  result  from  the  body  pro- 
duced from  it,  which  changes  rapidly  again  with  a 
period  of  only  a  few  days.  We  shall  pass  over  these 
intermediate  changes  and  consider  the  last  known 
change  of  the  series,  that  of  radium  F,  which  has 
a  period  of  average  life  of  202  days,  in  which  an 
a-particle  is  expelled.  Radium  F  is  the  polonium 


POLONIUM  205 

of  Madame   Curie,   having  been  separated   by   her 
from  pitchblende  first  before  she  discovered  radium. 

A  digression  may  here  conveniently  be  made  on 
what  is  known  about  polonium,  before  its  connec- 
tion with  radium  is  considered.  Chemically  it  re- 
sembles bismuth,  and  was  separated  first  from 
pitchblende  in  association  with  the  bismuth  con- 
tained in  the  mineral.  Its  radioactivity,  which 
consists  entirely  of  a-rays,  slowly  and  completely 
decays,  so  that  a  few  years  after  it  has  been  pre- 
pared, the  most  intensely  active  preparations  of  it 
lose  practically  all  their  activity.  The  work  was 
carried  on  by  Marckwald  in  Germany,  who  dis- 
covered new  and  simple  methods  of  extracting 
polonium  from  the  mineral  and  worked  up  many 
tons  of  pitchblende  for  this  substance.  His  careful 
chemical  investigations  of  the  nature  of  the  body 
made  it  clear  that  it  was  quite  as  nearly  allied  in 
chemical  nature  to  the  element  tellurium  as  to  bis- 
muth, and  he  first  proposed  the  name  "radio- 
tellurium  "  for  it,  which,  however,  with  the  elucidation 
of  its  identity  with  polonium,  has  fallen  into  disuse. 
He  proved  that  there  is  far  less  polonium  in  the 
mineral  even  than  radium.  In  a  ton  of  mineral 
there  is  less  than  a  thousandth  part  of  a  grain  of 
polonium,  but  the  radioactivity  is  correspondingly 
intense,  and  greatly  exceeds,  so  far  as  the  a-radiation 


206  POLONIUM 

is  concerned,  that  of  pure  radium  itself.  The  period 
of  average  life,  202  days,  is  deduced  by  direct 
observation  from  the  rate  of  decay  of  the  radio- 
activity. 

Returning  now  to  the  consideration  of  radium  C, 
we  saw  that  after  its  activity  had  decayed  there 
existed  still  a  residual  activity  which  is  very  feeble. 
This  steadily  increases  with  time,  and  consists  both 
of  «-  and  /5-rays,  which,  however,  increase  at  dif- 
ferent rates.  The  a-rays  are  due  to  polonium,  or 
radium  F.  These  go  on  increasing  for  the  first 
two  years  and  then  a  maximum  is  reached,  the 
amount  of  the  radium  F  formed  being  in  equilibrium. 
The  /3-rays,  however,  reach  a  maximum  much 
more  quickly.  The  /3-ray  product  (radium  E) 
having  a  much  shorter  period,  equilibrium  is  reached 
in  a  few  weeks.  If  at  any  time  the  active  matter 
is  subjected  to  the  chemical  processes  worked  out 
by  Marckwald  for  the  separation  of  polonium,  the 
a-ray  body  radium  F  can  be  separated  from  the 
other  products,  and  its  activity  then  decays  away 
completely  at  exactly  the  same  rate  as  in  the  case 
of  polonium.  Moreover,  it  shows  the  property  of 
being  volatile  at  a  temperature  of  a  bright  red  heat, 
which  is  the  basis  of  one  of  the  methods  originally 
used  by  Madame  Curie  in  separating  polonium 
from  the  bismuth  in  pitchblende.  This  is  merely 


WHAT  IS  THE    ULTIMATE  PRODUCT?         207 

a  sketch  of  the  evidence  in  favour  of  regarding 
polonium  as  the  last  radioactive  substance  produced 
in  the  disintegration  of  uranium. 

t 
One  more  step  remains  to  be  discussed,  and  then 

this  long  story  of  continuous  transformation  is  at 
an  end.  What  is  the  ultimate  product  ?  When 
radium  F  or  polonium  expels  its  a-particle,  what 
is  produced  ?  The  estimated  atomic  weight  of 
polonium  is  210*5,  which  is  deduced  by  subtracting 
from  the  atomic  weight  of  uranium  (238*5)  the 
weight  of  the  seven  atoms  of  helium  known  to  be 
expelled  in  the  form  of  a-particles.  This  agrees 
well  with  its  chemical  nature,  for  there  is  a  vacant 
place  in  the  periodic  table  for  an  element,  the  next 
heavier  than  bismuth  (atomic  weight,  208*5),  and 
this  element  would  be  chemically  analogous  to 
tellurium.  The  expulsion  of  an  a-particle  would 
further  reduce  the  atomic  weight  four  units,  leaving 
a  residue  of  atomic  weight  206*5.  What  is  it  ? 

Now,  if  this  is  really  the  final  product  and  not 
merely  a  very  slowly  changing  substance,  the  forma- 
tion of  which  in  proportion  to  the  degree  of  slow- 
ness of  the  change  would  be  difficult  experimentally 
to  detect,  then  it  follows  that  the  ultimate  product 
must  accumulate  in  quantity  indefinitely  with  time 
in  the  minerals  containing  the  elements  of  the 
uranium-radium  series,  and  must  therefore  be  a 
well-known  common  element.  There  seems  to  be 


208  LEAD  IN  RADIUM  MINERALS 

only  a  possible  choice  of  two  such  elements.  Lead 
has  the  atomic  weight  of  207*1,  and  bismuth,  208*0. 
The  next  known  element  is  thallium  (204),  and  then 
comes  mercury  (200). 

For  long  it  has  looked  as  if  lead  would  prove  to 
be  the  ultimate  product.     It  must  be  remembered 
that  the  atomic  weight  of  helium  is  not  very  accu- 
rately known,  and  is  probably  a  little  below,  rather, 
than  above,  4.     So  that  the  atomic  weight  of  lead, 
207*1,  agrees  fairly  well  with  the  estimate  206*5,  ob- 
tained by  subtracting   from    the  atomic  weight  of 
radium  226*5,  the  weight  of  the  five  a-particles,  or 
helium  atoms,  known  to  be  expelled.      Lead  is  found 
in  all  the  common  minerals  containing  uranium  in 
considerable  quantity,  and  there  is  some  evidence 
that  the  older  the  geological  formation  from  which 
the  mineral  is  obtained,  the  greater  the  percentage 
of    lead   present.       Recently    a    uranium    mineral, 
autunite,  has  been  found  containing  no  chemically 
detectable   quantity  of  lead.     But   then    the   same 
mineral  contains  only  an  excessively  minute  trace  of 
helium,  and  less  than  its  full  equilibrium  amount  of 
radium.     There  is  every  reason  to  believe  that  its 
formation  as  a  mineral  has  occurred  in  quite  recent 
times. 

This  question  is  still  unsettled.  We  may  rest 
assured  that  the  patient  application  of  the  methods 
which  have  already  resulted  in  the  brilliant  solution 
of  many  problems  in  radioactivity,  at  first  apparently 


DISINTEGRATION  SERIES  OF  URANIUM    209 


insoluble  except  by  researches  extending  over  im- 
possible periods  of  time,  will  cause  this  last  strong- 
hold to  capitulate.  The  only  method  of  settling  it 
is  to  study  the  change  of  polonium,  separated  from 
enormous  quantities  of  pitchblende,  by  the  aid  of 
the  spectroscope,  and  on  this  task  Mme.  Curie  and 
her  colleagues  have  for  long  been  engaged. 


Uranium  I.  Uranium  II.  Uranium  X. 
8,000,000,000  2,000,000       35-5  days, 
years.  years  (?) 


Emanation.    Radium  A.    Radium  B.    Radium  C. 
5-6  days. 


Active  deposit  of  rapid  change. 
4-3  38'*  28-1 


minutes.       minutes.        minutes. 


Radium  D.    Radium  E.    Radium  F.    Radium  G. 
(Polonium.)      (Lead?) 

Active  deposit  of  slow  change. 
24  years  (?)     7*25  days.       202  days. 

FIG.  31. 

Fig.  31   shows,  so  far  as  it  is  at  present  known, 
the  complete  disintegration  series  of  uranium, 
p 


CHAPTER  X. 

Ratio  of  quantities  of  polonium  and  radium  in  minerals— Table  of  the 
ratio  of  the  quantities  of  all  the  products  of  uranium — Difficulty  of 
concentrating  many  of  the  long-lived  products  of  uranium — Increase 
of  activity  of  radium  with  time— Radioactivity  a  physical  measure 
of  value  or  rarity — The  currency  metals  and  their  rarity — Are  they 
changing  like  radium  ? — Physical  necessity  for  rarity  of  a  changing 
element — One  aspect  of  the  ultimate  nature  of  matter — A  quota- 
tion from  Clerk  Maxwell — Evolution  of  the  elements  denied — 
Similarity  of  all  the  atoms  of  the  same  element — The  atom  a 
complex  and  perfect  piece  of  mechanism — Professor  Schuster's 
analogy — The  atom  true  to  its  character  at  dissolution — Similarity 
in  the  velocity  of  all  a  particles  expelled  from  a  radio-element — 
The  more  stable  the  radio -element  the  slower  the  a-rays  expelled 
from  it — Survival  of  the  fittest  or  most  stable  atoms — Universality 
of  the  conception  of  evolution  to  the  material  universe,  animate 
and  inanimate. 

FROM  the  law,  which  has  already  been  found 
so  useful,  we  can  calculate  the  ratio  of  the 
quantities  of  radium  and  polonium  that  exist 
together  in  a  mineral  from  their  periods  of  average 
life.  The  period  of  average  life  of  radium  is  4500 
times  that  of  polonium,  so  that  there  must  be  4500 
times  more  radium  than  polonium  in  minerals.  A 
good  pitchblende  contains  about  an  ounce  of  radium 
in  150  tons.  The  same  quantity  of  polonium  would 
therefore  be  contained  in  about  700,000  tons.  The 
whole  output  of  the  Joachimsthal  mine  per  annum, 
reckoned  as  15  tons,  contains  about  one  hundredth 


210 


THE  LA  W  OF  PROPORTIONALITY 


21  I 


of  a   grain    of  polonium.     This    is    borne    out    by 
Marckwald's  experiments,  already  referred  to. 

Let  us  apply  the  law  not  only  to  radium  and 
polonium,  but  to  the  whole  list  of  known  transition- 
forms  existing  as  products  of  uranium.  In  the  table 
this  has  been  done.  The  first  column  gives  the 
name  of  the  substance,  the  second  its  period  of 
average  life,  and  the  third  its  relative  quantity  in 
minerals,  the  quantity  of  uranium  being  considered 
1,000,000,000.  If  these  numbers  are  taken  to  refer 
throughout  to  milligrams  (i  milligram  is  about  -^  of 
a  grain),  then  since  1,000,000,000  milligrams  is 
roughly  a  ton,  the  quantities  refer  to  an  amount  of 
mineral  containing  one  ton  of  the  element  uranium. 


TABLE. 


PERIOD. 


Uranium  I,  8,000,000,000  years. 
Uranium  II,  2,000,000  years  (?). 


Uranium  X  .. 
Ionium 
Radium 

Emanation  .. 

Radium  A  .. 

Radium  B  .. 
Radium  C 

Radium  D  .. 

Radium  E  .. 

Radium  F  .. 
(Polonium) 


35'5  days. 
200,000  years  (?). 
2,500  years. 
5-6  days. 
4'3  minutes. 
38*5  minutes. 
28-1  minutes. 
24  years. 
7-5  days. 
202  days. 


QUANTITY. 

1,000,000,000  mg.  (=  i  ton). 

250  grams  (?). 

One  eightieth  mg. 

25  grams  (?). 

312-5  mg. 

One  five-hundredth  mg. 

One  millionth  mg. 

Nine  millionths  mg. 

Seven  millionths  mg. 

3  mg. 

One  four-thousandth  mg. 

One  fourteenth  mg. 


212  DIFFICULTY  OF 

These  respective  quantities  in  the  last  column 
emit  a  similar  number  of  a-particles  per  second  in 
the  six  cases  where  a-particles  are  expelled  at  all, 
and  so  produce  similar  radioactive  effects.  This  is 
an  illustration  of  the  compensating  principle  I  spoke 
of  earlier,  that  the  quantity  of  a  radioactive  sub- 
stance divided  by  its  life,  not  the  quantity  only, 
gives  a  measure  of  its  radioactive  effects.  It  can 
readily  be  calculated  that  the  actual  amount  of 
radium  A  used  in  our  experiments,  which  produced 
powerful  and  striking  effects  on  the  phosphorescent 
screen,  was  much  below  one  ten-millionth  of  a 
milligram,  or  below  one  thousand-millionth  of  a 
grain.  For  it  was  derived  from  30  mg.,  i.e.  half  a 
grain  of  radium  bromide.  Yet  while  it  lasts  it 
comes  into  evidence  through  the  energy  of  the 
a-particles  expelled  in  its  rapid  disintegration  no 
less  than  any  of  the  other  products. 

The  table  brings  out  clearly  that  radium  is  but 
one  of  many  radioactive  substances  in  uranium 
minerals,  which  would  be  of  value  if  they  could  be 
extracted.  Uranium  II,  ionium  and  radium  D,  all 
possess  sufficiently  extended  periods  of  life  to  repay 
recovery.  Ionium  gives  only  very  feebly  penetrating 
a-rays,  and  so  would  not  be  so  generally  useful  as 
radium,  whereas  uranium  II  and  radium  D  both, 
being  followed  by  short-lived  products  which  give 
/3-rays,  would  be  of  great  general  utility.  The 


SEPARATING  RADIO-ELEMENTS  213 

reason  which  has  precluded  the  practical  separation 
of  these  substances  in  the  past  is  a  general  one, 
which  undoubtedly  is  of  the  highest  philosophical 
significance  in  the  chemistry  of  these  new  ephemeral 
elements.  They  all  so  closely  resemble  one  or 
other  of  the  known  elements  that  the  separation  is 
impossible.  The  resemblance  between  radium  and 
barium  is  of  great  practical  utility,  because  these 
two  elements,  though  very  closely  alike  in  chemical 
nature,  can  be  separated  from  each  other  after  they 
have  first  been  separated  from  every  other  element. 
Taking  them  in  order,  uranium  II  cannot  yet  be 
separated  from  uranium  I,  ionium  cannot  be  separ- 
ated from  thorium,  nor  radium  D  from  lead.  Lead, 
as  has  been  stated,  is  almost  always  present  in 
considerable  quantity  in  uranium  minerals,  and  so 
usually  is  thorium,  but  to  a  much  more  variable 
extent.  Hence,  though  it  is  easy  to  separate 
radium  D  from  the  mineral  with  the  lead,  it  is  at 
present  useless  practically,  as  it  cannot  be  con- 
centrated from  the  lead.  By  choosing  suitable 
minerals  like  secondary  pitchblendes,  which  do  not 
contain  ponderable  quantities  of  thorium,  intensely 
active  preparations  of  ionium  can  however  be 
separated.  It  is  at  present  the  only  one  in  the 
uranium  series  likely  to  become  useful,  and  its 
lack  of  penetrating  rays  is  a  serious  drawback. 
Polonium,  with  its  period  of  less  than  a  year  and 


2i4  INCREASE   OF  ACTIVITY 

its  absence  of  penetrating  rays,  hardly  repays  ex- 
traction, except  for  purely  scientific  investigations. 
There  is,  however,  another  disintegration  series, 
that  of  thorium,  which  offers  a  better  chance  of 
providing  an  efficient  substitute  for  radium,  and 
this  series  will  therefore  be  briefly  considered  in 
the  concluding  chapter. 

The  increase  of  the  radioactivity  of  radium  after 
it  is  prepared  is  due  to  the  steady  growth  of  the 
products  undergoing  further  disintegration.  As 
we  know,  when  freshly  prepared  from  solution, 
the  activity  of  radium  is  due  solely  to  its  own 
disintegration  and  consists  of  a-rays.  After  four 
weeks  the  first  four  products  accumulate  to  their 
equilibrium,  and  the  activity  now  consists  of 
a-,  /3-,  and  7-rays,  the  a-rays  being  four  times  as 
great  as  initially.  It  is  not  difficult  to  see  that  the 
later  slow  changes  must  also  cause  a  very  slow 
further  continuous  increase  of  all  these  types  of 
rays.  There  is  reason  to  believe  that  the  change 
of  radium  C  is  double,  two  /3-particles  and  one 
a-particle  being  expelled  in  the  double  change,  but 
that  is  not  yet  settled.  These  considerations  are 
embodied  in  the  following  table  giving  an  analysis 
of  the  total  radioactivity  of  a  radium  preparation, 
kept  in  a  sealed  vessel  so  that  none  of  the  products 
escape,  at  different  periods  since  preparation  : — 


OF  RADIUM  WITH  AGE  215 

a-PARTICLES,  /9-PARTICLES. 

I.  Freshly  prepared,    i  (due  to  radium  itself )  o 

II.  After  one  month.    4  (i  due  to  radium)  i  or  2 

(i  due  to  emanation)  (due  to  Ra  C) 
(i  due  to  radium  A) 
(i  due  to  radium  C) 

III.  After  a  century.      5   (as  in  II  and  i  due  2  or  3 

to   radium  F)  (i  due  to  Ra  E2) 

The  idea,  which  is  a  necessary  consequence  of 
the  atomic  disintegration  theory,  that  fixed  definite 
relationships  must  exist  between  the  quantities  of 
elements  formed  from  one  another — for  example, 
between  uranium,  radium,  and  polonium — forms  the 
first  indication  that  physical  laws  may  exist  regulat- 
ing the  relative  abundance  and  scarcity  of  elements 
in  Nature.  Gold  and  platinum,  for  example,  are 
valuable  or  rare  metals,  and  we  do  not  know  why. 
Radioactive  bodies  like  radium  are  rare  because  of 
the  rapidity  with  which  they  are  changing.  The 
degree  of  radioactivity  of  an  element  being  pro- 
portional to  the  rate  at  which  it  is  changing,  it 
follows  that  radioactive  elements  are  scarce  and 
valuable  in  proportion  to  their  radioactivity.  In 
this  case  degree  of  radioactivity  is  a  physical 
measure  of  value  or  rarity.  It  is,  for  example, 
so  far  as  we  can  see,  an  impossibility  that  an 
element  like  radium  will  ever  be  found  in  greater 
abundance  in  any  minerals  than  in  those  already 
known. 


2x6         A  PHYSICAL  MEASURE   OF  RARITY 

Naturally,  in  the  consideration  of  some  of  these 
questions  of  general  interest  upon  which  we  are 
now  entering,  we  are,  be  it  said,  in  sharp  contrast 
to  almost  everything  we  have  dealt  with  in  the 
subject  up  to  now,  frankly  speculating.  But  it  is 
helpful  and  legitimate  to  speculate  upon  how  far, 
if  at  all,  the  process  of  atomic  disintegration,  dis- 
covered for  the  radio-elements,  applies  to  the  case 
of  elements  not  radioactive,  of  which  there  is  as  yet 
no  positive  evidence  that  they  are  changing  at  all. 
The  workers  in  radioactivity  have  within  their 
province  explored  thoroughly  the  process  of  atomic 
disintegration.  They  have  made  clear  the  laws  it 
follows,  they  have  measured  the  rates  at  which 
it  occurs,  and  they  have  established  what  may  be 
termed  its  inevitableness  or  independence  from  all 
known  influences.  But  there  is  no  reason  why  the 
process  should  be  limited  in  its  scope  to  the  some- 
what special  phenomena  which  led  to  its  discovery. 

It  is,  for  example,  natural  to  inquire  whether  the 
scarcity  of  elements  like  gold  is  fixed  by  the  opera- 
tion of  similar  physical  laws  to  those  which  regulate 
the  rarity  of  radium.  The  race  has  grown  used 
from  the  earliest  times  to  the  idea  that  gold  is  a 
metal  possessing  a  certain  fixed  degree  of  value, 
enabling  it  to  be  used  safely  for  the  purposes  of 
currency  and  exchange.  It  is  no  exaggeration  to 


THE  GOLD  CURRENCY  217 

say  that  the  whole  social  machinery  of  the  Western 
world  would  be  dislocated  if  gold  altered  violently 
in  its  degree  of  rarity — if,  for  example,  in  some 
hitherto  unpenetrated  fastness  of  the  globe  a  moun- 
tain of  gold  came  to  be  discovered.  Is  there  not 
at  least  a  strong  presumption  that  this  is  really  as 
contrary  to  the  operation  of  natural  law  as  the  dis- 
covery of  a  mountain  of  pure  radium  would  be  ? 

It  may,  I  think,  be  taken  for  granted  that  an 
element  changing  more  rapidly  than  uranium,  for 
example, — that  is  with  a  period  of  average  life  of 
less  than  8,000,000,000  years — is  not  likely  to  be 
much  more  plentiful  in  nature  than  uranium,  and 
therefore  that  all  the  common  elements — lead, 
copper,  iron,  oxygen,  silicon,  etc.  etc. — have 
periods  of  average  life  of  many  thousands  of 
millions  of  years.  So  far,  the  traditional  view 
that  the  elements  are  permanent  and  unchanging 
is  substantially  correct.  At  the  same  time,  we 
cannot  but  recognise  that  inevitably  the  effects 
of  atomic  disintegration,  too  slow  to  be  other- 
wise detectable,  would  result  in  the  accumulation 
of  the  more  stable  and  longest-lived  elements  at 
the  expense  of  the  others,  resulting  in  some  sort 
of  equilibrium  in  which  the  relative  abundance  of 
the  elements  was  proportional  to  their  respective 
periods  of  average  life.  For  example,  the  ratio 
between  the  relative  abundance  of  gold  and  silver 


2iS       RELATIVE  ABUNDANCE   OF  ELEMENTS 

is  roughly  but  pretty  certainly  known,  owing  to 
these  metals  being  employed  for  currency  purposes 
from  the  earliest  times.  It  is  at  least  a  possible 
view  to  take  that  the  elements  gold  and  silver 
belong  to  the  same  disintegration  series,  both 
changing  very  slowly,  but  the  gold  many  times 
more  rapidly  than  the  silver.  Obviously  we  are 
only  at  the  beginning.  But  already  it  cannot  be 
gainsaid  that  the  interest  and  importance  of  this 
process  of  atomic  disintegration  is  not  confined  to 
radioactivity  only  or  even  to  physical  science.  It 
extends  into  almost  every  region  of  thought. 

This  is  the  chief  reason,  and  must  be  my  excuse 
for  being  so  bold  as  to  attempt  to  bring  before 
a  general  audience  so  much  of  the,  in  one  sense, 
highly  technical  discoveries  of  radioactivity.  It 
would  be  a  pity  if  mere  technicalities  and  strange 
words  barred  the  progress  of  ideas  and  their 
application  to  everyday  thought.  Otherwise  the 
scientific  discoverer  would,  so  far  as  his  own  times 
were  concerned,  labour  half  in  vain,  and  worse,  the 
accurate  and  complete  application  of  his  work  to 
other  fields  of  thought  would  be  delayed  by  erro- 
neous and  partial  ideas,  mere  half-truths,  springing 
up  in  their  place. 

I  now  propose  considering  briefly  another  ques- 
tion of  general  philosophical  interest  in  connection 


THE  NATURE   OF  ATOMS  219 

with  the  recent  advances  of  physical  science. 
Naturally  the  discoveries  in  radioactivity  have  not 
been  made  without  influencing  considerably  our 
ideas  on  the  ultimate  nature  of  atoms.  In  some 
points  older  conceptions  have  had  to  be  modified, 
while  in  others  these  conceptions  have  been 
strangely  confirmed.  It  has  always  been  a  matter 
for  remark,  considering  the  myriads  of  individual 
atoms  which  go  to  make  up  the  smallest  perceptible 
quantity  of  matter,  that  there  are  so  few  different 
kinds.  The  number  of  atoms  which  go  to  make 
up  this  world,  for  example,  would  run  into  at  least 
fifty-four  figures,  yet  among  them  all  there  are  less 
than  a  hundred  different  varieties.  Moreover,  it 
has  come  to  be  regarded  as  one  of  the  greatest 
philosophical  generalisations  of  physical  science 
that  all  the  atoms  of  one  kind,  that  is  to  say  of 
one  element,  are  exactly  and  completely  similar  in 
character.  There  is,  for  example,  not  the  shadow 
of  distinction  between  gold  found  in  the  Klondyke, 
in  Australia,  or  in  S.  Africa.  Not  only  so,  but 
we  have  learned  from  the  spectroscope  that  this 
similarity  of  nature  extends  throughout  the  whole 
universe.  In  this  connection/both  to  set  forth  the 
idea  and  to  illustrate  the  deductions  which  have 
been  drawn  from  it,  I  cannot  do  better  than  to 
quote  a  celebrated  utterance  of  Clerk  Maxwell  to 
the  British  Association  in  1873.  I  may  remark 


220       A    QUOTATION  FROM  CLERK  MAXWELL 

that  Clerk  Maxwell  throughout  used  the  word 
molecule  in  the  sense  of  "atom"  as  this  word  is 
employed  by  the  chemist,  and  throughout  these 
lectures. 

"  In  the  heavens  we  discover  by  their  light,  and 
by  their  light  alone,  stars  so  far  distant  from  each 
other  that  no  material  thing  can  ever  have  passed 
from  one  to  another  ;  and  yet  this  light,  which  is 
to  us  the  sole  evidence  of  the  existence  of  these 
distant  worlds,  tells  us  also  that  each  of  them  is 
built  up  of  molecules  of  the  same  kinds  as  those 
which  we  find  on  earth.  A  molecule  of  hydrogen, 
for  example,  whether  in  Sirius  or  in  Arcturus, 
executes  its  vibrations  in  precisely  the  same  time. 

"  Each  molecule  therefore  throughout  the  uni- 
verse bears  impressed  upon  it  the  stamp  of  a  metric 
system  as  distinctly  as  does  the  metre  of  the 
Archives  at  Paris,  or  the  double  royal  cubit  of  the 
temple  of  Karnac. 

"  No  theory  of  evolution  can  be  formed  to 
account  for  the  similarity  of  molecules,  for  evolu- 
tion necessarily  implies  continuous  change,  and  the 
molecule  is  incapable  of  growth  or  decay,  of 
generation  or  destruction. 

"  None  of  the  processes  of  Nature,  since  the 
time  when  Nature  began,  have  produced  the 
slightest  difference  in  the  properties  of  any  mole- 
cule. We  are  therefore  unable  to  ascribe  either 
the  existence  of  the  molecules  or  the  identity  of 


FOUNDATION-STONES  OF  THE  UNIVERSE     221 

their  properties  to  any  of  the  causes  which  we  call 
natural. 

"  On  the  other  hand,  the  exact  equality  of  each 
molecule  to  all  the  others  of  the  same  kind  gives 
it,  as  Sir  John  Herschel  has  well  said,  the  essential 
character  of  a  manufactured  article,  and  precludes 
the  idea  of  its  being  eternal  and  self-existent. 

"  Thus  we  have  been  led,  along  a  strictly  scien- 
tific path,  very  near  to  the  point  at  which  science 
must  stop  ;  not  that  science  is  debarred  from  study- 
ing the  internal  mechanism  of  a  molecule  which 
she  cannot  take  to  pieces,  any  more  than  from 
investigating  an  organism  which  she  cannot  put 
together.  But  in  tracing  back  the  history  of 
matter,  Science  is  arrested  when  she  assures  herself, 
on  the  one  hand,  that  the  molecule  has  been  made, 
and  on  the  other,  that  it  has  not  been  made  by  any 
of  the  processes  we  call  natural. 

"  Science  is  incompetent  to  reason  upon  the 
creation  of  matter  itself  out  of  nothing.  We  have 
reached  the  utmost  limits  of  our  thinking  faculties 
when  we  have  admitted  that  because  matter  cannot 
be  eternal  and  self-existent  it  must  have  been 
created." 

You  will  admit  that,  in  the  light  of  all  that  has 
transpired  in  the  thirty-five  years  since  Maxwell 
used  these  words,  science  has  advanced  far.  The 
concluding  words  of  the  address  are  even  more 
striking  from  this  point  of  view. 


222  COMPLEXITY  OF  ATOMS 

"  Natural  causes,  as  we  know,  are  at  work,  which 
tend  to  modify,  if  they  do  not  at  length  destroy,  all 
the  arrangements  and  dimensions  of  the  earth  and 
the  whole  solar  system.  But  though  in  the  course 
of  ages  catastrophes  have  occurred  and  may  yet 
occur  in  the  heavens,  though  ancient  systems  may 
be  dissolved  and  new  systems  evolved  out  of  their 
ruins,  the  molecules  out  of  which  these  systems  are 
built — the  foundation-stones  of  the  material  universe 
— remain  unbroken  and  unworn." 

Before  we  dwell  upon  the  modifications  that  have 
been  made  in  this  point  of  view,  let  us  rather  con- 
sider the  chief  basis  of  the  argument,  namely,  that 
all  the  atoms  of  any  one  element  are  exactly  alike. 
On  this  fundamental  question  the  evidence  to-day 
is  far  more  complete  and  striking  than  it  was  in 
1873,  and  we  believe  more  firmly  than  ever  in  the 
absolute  similarity  of  all  the  atoms  of  the  same 
element. 

We  no  longer  regard  the  atom  as  a  simple  thing. 
On  the  contrary,  we  now  look  upon  it  as  an  almost 
infinitely  complex  piece  of  mechanism.  The  late 
Professor  Rowland,  of  Baltimore,  once  made  the 
remark  that  a  grand  piano  must  be  a  very  simple 
piece  of  mechanism  compared  with  an  atom  of 
iron.  For  in  the  spectrum  of  iron  there  is  an 
almost  innumerable  wealth  of  separate  bright  lines, 
each  one  of  which  corresponds  to  a  sharp  definite 
period  of  vibration  of  the  iron  atom.  Instead  of 


AND  THEIR  SIMILARITY  223 

the  hundred-odd  sound  vibrations  which  a  grand 
piano  can  emit,  the  single  iron  atom  appears  to 
emit  many  thousands  of  definite  light  vibrations. 
Two  pianos  would  be  regarded  as  in  perfect  tune 
together  when  there  was  a  comparatively  rough 
approximation  of  period  between  the  various  notes. 
Whereas  by  the  spectroscope  a  difference  in  "  tune  " 
or  period  in  the  vibrations  emitted  by  different 
atoms  of  only  one  part  in  many  millions  would  be 
easily  detectable,  and  no  such  variation  exists.  In 
a  similar  vein  Professor  Schuster,  referring  to  the 
broad  teachings  of  the  spectroscope,  has  compared 
the  atoms  of  the  same  element  to  an  innumerable 
number  of  clocks  all  wound  and  regulated  to  go  at 
the  same  period.  If  all  these  clocks  were  set  at  the 
same  time,  not  one  of  them  would  vary  by  a  single 
second  even  after  many  days.  No  clockmaker 
could  make  such  clocks.  Yet  these  almost  infinitely 
complicated  pieces  of  mechanism  we  call  atoms  are 
turned  out  by  Nature  with  such  undeviating  accu- 
racy and  fidelity  that  in  all.  the  myriads  in  existence 
there  are  less  than  a  hundred  different  kinds 
known. 

We  can,  however,  from  the  point  of  view  of 
recent  researches  in  radioactivity,  push  this  idea 
even  one  step  further,  to  the  case  of  atoms  actually 
in  the  condition  of  breaking  up.  We  have  seen 
that  it  is  a  property  of  the  a-rays  to  possess  a 


224  VELOCITIES  OF  THE  ^-PARTICLES 

very  sharp  and  definite  range.  In  a  beam  of 
homogeneous  a-rays  passing  through  a  homoge- 
neous absorbing  medium  the  number  of  a-particles 
suffers  little  or  no  diminution  until  the  extreme 
end  of  the  path  is  reached,  and  then  they  cease 
altogether.  Just  without  the  extreme  range,  there 
is  absolutely  no  effect  perceptible,  while  just  within 
this  range,  the  effect,  per  small  element  of  path,  is 
at  the  maximum.  Every  a-particle  expelled  from 
the  radio-element  in  the  same  change  travels 
exactly  the  same  distance  before  it  ceases  to  be 
detectable,  and,  as  Rutherford  has  shown  by  direct 
measurement  of  the  magnetic  and  electric  devia- 
tion, is  expelled  at  the  same  velocity. 

In  the  table  following,  the  approximate  initial 
velocities  of  the  a-particles  from  the  changes  in  the 
uranium  series  have  been  collected,  together  with 
their  " ranges"  or  distances  in  millimetres  they  will 
penetrate  in  air  at  15°  C.  and  760  mm.  of  mercury 
pressure. 

VELOCITY 
a-PARTiCLE  FROM  (miles  per  second).  RANGE. 

Uranium  I 8,800  25 

Uranium  II 9*300  29 

Ionium          ...         ...         9  A00  •••         •••         3° 

Radium         ...         .,.          9,600  ...  33 

Emanation 10,400  42 

Radium  A     10,900  47 '5 

Radium  C     12,400  69-5 

Radium  F     10,200  ...  377 

The   atom    thus    retains    its    role    of    a   perfect 


MATERIAL  EVOLUTION  225 

piece  of  mechanism  even  up  to  and  during  the 
moment  of  its  dissolution.  So  exactly  alike  are 
all  the  atoms  of  the  same  radioactive  element,  that 
when  the  break-up  occurs  the  velocity  with  which 
the  fragments  of  the  atom,  or  ^-particles,  are  ex- 
pelled is  exactly  the  same  in  each  case.  We  may 
liken  the  disintegration  of  an  element  to  the  burst- 
ing of  shells,  in  which  the  fragments  of  the  different 
shells  all  are  expelled  with  the  same  velocity. 
Certainly  no  shells  ever  constructed  would  answer 
this  requirement.  Truly,  in  the  words  of  Sir  John 
Herschel,  the  atom  bears  the  essential  character  of 
a  manufactured  article,  but  of  a  degree  of  per- 
fection humanly  unattainable. 

But  with  regard  to  the  process  of  manufacture 
and  of  the  cause  of  this  undeviating  fidelity  to  a 
few  types,  what  a  revolution  of  thought  has  taken 
place  in  the  last  few  years !  The  evolution,  or 
rather  devolution,  of  matter,  its  continuous  change, 
the  generation  and  destruction  of  atoms — all  of 
the  things  which  seemed  impossible  in  Clerk  Max- 
well's day — we  know  to  be  going  on  before  our  eyes. 
It  is  true  the  processes  call  for  periods  of  time 
so  vast,  even  in  the  most  favourable  cases,  that 
the  physicist  of  a  generation  ago  would  have 
dismissed  them  as  physically  inconceivable.  Yet 
these  periods  are  to-day  actually  determined  by 

direct  measurement  in  the  laboratory. 
Q 


226  CONNECTION  BETWEEN  PERIOD 

Before  leaving  this  question  a  very  interesting 
development  may  be  mentioned,  which  has  resulted 
in  a  connection  being  established  between  the  ranges 
or  velocities  of  the  various  types  of  «-rays,  and  the 
periods  of  life  of  the  atoms  from  which  they  are 
derived.  As  a  general  rule — not,  it  is  true,  entirely 
without  exceptions,  but  possibly  the  exceptions  may 
prove  to  be  only  apparent — the  more  rapidly  a 
radioactive  substance  disintegrates,  or  the  shorter 
its  period  of  average  life,  the  greater  is  the  velocity 
with  which  the  a-particle  is  expelled  from  the  atom, 
and  the  greater  therefore  is  the  range  of  the  a-particle. 
Thus,  the  most  stable  radio-elements,  uranium  and 
thorium,  give  a-rays  having  the  lowest  ranges,  and 
the  low  range  of  the  a-rays  of  ionium  is  additional 
evidence  that  its  period  must  be  very  long.  The 
greatest  ranges  occur  in  the  short-lived  "  active 
deposit"  products.  The  very  long  ranges  of  the 
a-rays  of  radium  C  (69*5  mm.),  and  of  the  corres- 
ponding thorium  C  (86  mm.),  is  generally  explained 
by  the  supposition  that  the  real  atoms  giving  these 
rays  have  periods  of  the  order  of  only  a  millionth 
of  a  second,  and  therefore  that  it  is  impossible  to 
separate  them  from  their  parents,  which  thus  appear 
to  be  giving  rays  which  in  reality  come  from  their 
products.  Two  actual  cases  of  such  a  sequence  will 
be  considered  in  the  last  chapter.  Latterly,  this 
generalisation  has  been  put  into  stricter  form  by  the 


AND  RANGE  -DF  a-RA  YS  227 

discovery  that  if  the  logarithm  of  the  period  is  plotted 
against  the  logarithm  of  the  range  or  of  the  velocity, 
straight  lines  result  for  each  of  the  three  known 
disintegration  series.  The  three  straight  lines  are 
parallel  to  but  not  identical  with  one  another. 
The  reason  for  this  is  still  obscure.  Some  mathe- 
matical connection  exists  between  the  two  quantities, 
and  that  is  all  that  can  yet  be  said.  On  the  other 
hand,  it  has  been  found  possible  to  calculate  some 
of  the  unknown  periods — like  that  of  ionium,  so 
estimated  at  200,000  years,  for  example — from  the 
ranges  of  the  a-rays  by  means  of  this  relation.  For 
long  it  was  known  that  uranium  was  exceptional 
in  that  it  appeared  to  give  out  two  a-particles  per 
atom  disintegrating  instead  of  one,  as  in  all  other 
cases.  A  very  careful  investigation  revealed  the 
fact  that  the  ranges  of  these  two  sets  of  a-particles 
were  not  exactly  alike.  One  set,  those  from 
uranium  I,  presumably,  have  a  range  of  25  mm., 
and  the  other  set,  those  from  its  shorter-lived 
product,  uranium  II,  presumably,  a  range  of  29  mm. 
The  period  corresponding  with  29  mm.  of  range 
is,  in  the  uranium  series,  two  million  years,  and 
this  is  the  main  evidence  for  believing  that  such 
a  product,  uranium  II  as  it  is  called,  exists,  and 
that  it  has  so  far  not  been  separated  from  uranium 
because  of  the  identity  of  the  chemical  properties 
of  the  two  elements. 


228  MATERIAL  EVOLUTION 

Instead  of  regarding  the  hundred  or  less  ele- 
ments which  exist  to-day  as  manufactured,  created, 
once  for  all  time,  we  rather  regard  them  as 
existing  because  they  have  survived.  All  other 
forms  less  stable  than  those  we  recognise  as 
elements  have  been  weeded  out.  Over  sufficiently 
great  periods  of  time  the  rarity  or  abundance  of 
an  element  must  be  controlled  by  its  degree  of 
instability  or  stability.  Probably  for  every  stable 
atom  many  unstable  ones  could  be,  even  are  being, 
formed.  But  only  the  stable  forms  can  accumu- 
late in  quantity  and  become  known  to  us  as 
ordinary  chemical  elements.  We  have  seen  that 
the  rarest  of  such  in  all  probability  must  have 
a  period  of  thousands  of  millions  of  years,  while 
for  the  more  common  elements,  if  they  are  chang- 
ing at  all,  periods  of  billions  of  years  may  be 
anticipated. 

At  first  glance  only,  the  material  universe  gives 
the  impression  of  a  permanent  and  finished 
creation.  In  reality  the  now  familiar  remorseless 
operation  of  slow,  continuous  change  moulds  even 
"the  foundation-stones"  themselves.  By  this  last 
step  the  doctrine  of  evolution  has  become  universal, 
embracing  alike  the  animate  and  inanimate  worlds. 
But  whereas  in  the  former  slight  changes  of 
environment  effect  the  profoundest  modifications, 
in  the  latter  the  controlling  factors  still  remain 


MATERIAL  EVOLUTION  229 

absolutely  unknown.  By  the  spectroscope  a  partial 
material  survey  of  the  whole  universe  has  been 
rendered  possible,  and  what  we  find  is  every- 
where an  essential  similarity  of  composition.  For 
example,  there  is  no  evidence  that  in  the  sun 
or  stars  large  quantities  of  elements  unknown  to 
us  exist.  The  reason  why  some  atoms  are 
stable  and  others  are  not  is  a  mystery  we  have 
not  yet  begun  to  probe.  Yet  this  question,  to 
us  only  of  academic  interest  and  possibly  some- 
what remote  at  that,  will,  as  we  shall  soon  come 
to  see,  be  one  of  life  and  death  to  the  inheritors 
of  our  civilisation. 


CHAPTER    XI. 

Why  is  radium  unique  among  the  elements  ? — Its  rate  of  change  only 
makes  it  remarkable — Uranium  is  more  wonderful  than  radium — 
The  energy  stored  up  in  a  pound  of  uranium — Transmutation  is 
the  key  to  the  internal  energy  of  matter — The  futility  of  ancient 
alchemy — The  consequences  if  transmutation  were  possible — 
Primitive  man  and  the  art  of  kindling  fire — Modern  man  and  the 
problem  of  transmutation — Cosmical  evolution  and  atomic  dis- 
integration— Radioactivity  and  geology — Quantity  of  radium  in 
the  earth's  crust — The  earth  probably  not  a  cooling  globe — The 
geological  age  and  the  incandescent  age — Ancient  mythology  and 
radioactivity — The  serpent  "  Ouroboros  " — The  "Philosopher's 
Stone "  and  the  "  Elixir  of  Life  "—The  "  Fall  of  Man  "  and  the 
"  Ascent  of  Man  " — The  great  extension  in  the  possible  duration 
of  past  time — Radium  and  the  struggle  for  existence — Existence 
as  a  struggle  for  physical  energy — The  new  prospect. 

THIS  interpretation  of  radium  is  drawing  to 
a  close,  but  perhaps  the  more  generally 
interesting  part  of  it  remains  to  be  dealt  with. 
We  have  steadily  followed  out  the  idea  of  atomic 
disintegration  to  its  logical  conclusions,  so  far 
as  they  can  at  present  be  drawn,  and  we  have 
found  it  able  to  account  for  all  the  surprising 
discoveries  that  have  been  made  in  radioactivity, 

and  capable  of  predicting  many,  and  perhaps  even 

230 


WHY  RADIUM  IS   UNIQUE  231 

more  unexpected,  new  ones.  Let  us  from  the 
point  of  vantage  we  have  gained  return  to  the 
starting  -  point  of  our  inquiries  and  see  what  a 
profound  change  has  come  over  it  since  the  riddle 
has  been  read.  Radium,  a  new  element,  giving 
out  light  and  heat  like  Aladdin's  lamp,  apparently 
defying  the  law  of  the  conservation  of  energy, 
and  raising  questions  in  physical  science  which 
seemed  unanswerable,  is  no  longer  the  radium 
we  know.  But  although  its  mystery  has  vanished, 
its  significance  and  importance  have  vastly  gained. 
At  first  we  were  compelled  to  regard  it  as  unique, 
dowered  with  potentialities  and  exhibiting  pecu- 
liarities which  raised  it  far  above  the  ordinary 
run  of  common  matter.  The  matter  was  the  mere 
vehicle  of  ultra-material  powers.  If  we  now  ask, 
why  is  radium  so  unique  among  the  elements,  the 
answer  is  not  because  it  is  dowered  with  any  ex- 
ceptional potentialities  or  because  it  contains  any 
abnormal  store  of  internal  energy  which  other 
elements  do  not  possess,  but  simply  and  solely 
because  it  is  changing  comparatively  rapidly, 
whereas  the  elements  before  known  are  either 
changing  not  at  all  or  so  slowly  that  the  change 
has  been  unperceived.  At  first  sight  this  might 
seem  an  anti-climax.  Yet  it  is  not  so.  The 
truer  view  is  that  this  one  element  has  clothed 
with  its  own  dignity  the  whole  empire  of  common 


232  WHY  JtADIUM  IS   UNIQUE 

matter.  The  aspect  which  matter  has  presented 
to  us  in  the  past  is  but  a  consummate  disguise,  con- 
cealing latent  energies  and  hidden  activities  beneath 
an  hitherto  impenetrable  mask.  The  ultra-material 
potentialities  of  radium  are  the  common  possession 
of  all  that  world  to  which  in  our  ignorance  we  used 
to  refer  as  mere  inanimate  matter.  This  is  the 
weightiest  lesson  the  existence  of  radium  has 
taught  us,  and  it  remains  to  consider  the  easy 
but  remorseless  reasoning  by  which  the  conclusion 
is  arrived  at. 

Two  considerations  will  make  the  matter  clear. 
In  the  first  place,  the  radioactivity  of  radium  at 
any  moment  is,  strictly  speaking,  not  a  property  of 
the  mass  of  the  radium  at  all,  although  it  is 
proportional  to  the  mass.  The  whole  of  the  new 
set  of  properties  is  contributed  by  a  very  small 
fraction  of  the  whole,  namely,  the  part  which  is 
actually  disintegrating  at  the  moment  of  observa- 
tion. The  whole  of  the  rest  of  the  radium  is  as 
quiescent  and  inactive  as  any  other  non-radioactive 
element.  In  its  whole  chemical  nature  it  is  an 
ordinary  element.  The  new  properties  are  not 
contributed  at  all  by  the  main  part  of  the  matter, 
but  only  by  the  minute  fraction  actually  at  the 
moment  disintegrating. 


THE  ENERG  Y  1-N  URANIUM  233 

Let  us  next  compare  and  contrast  radium  with 
its  first  product,  the  emanation,  and  with  its 
original  parent,  uranium.  Uranium  on  the  one 
hand,  and  the  emanation  on  the  other,  represent, 
compared  with  radium,  diametrically  opposed 
extremes.  Uranium  is  changing  so  slowly  that 
it  will  last  for  thousands  of  millions  of  years,  the 
emanation  so  rapidly  that  it  lasts  only  a  few  weeks, 
while  radium  is  intermediate  with  a  period  of 
average  life  of  two  thousand  five  hundred  years. 

We  have  seen  that  in  many  ways  the  emanation 
is  far  more  wonderful  than  radium,  as  the  rate  its 
energy  is  given  out  is  relatively  far  greater.  But 
this  is  compensated  for  by  the  far  shorter  time 
its  activity  lasts.  Also,  if  we  compared  uranium 
with  radium,  we  should  say  at  once  that  radium 
is  far  more  wonderful  than  the  uranium,  whereas 
in  reality  it  is  not  so,  as  the  uranium,  changing 
almost  infinitely  more  slowly,  lasts  almost  infinitely 
longer. 

The  arresting  character  of  radium  is  to  be 
ascribed  solely  to  the  rate  at  which  it  happens 
to  be  disintegrating.  The  common  element 
uranium,  well  known  to  chemists  for  a  century 
before  its  radioactivity  was  suspected,  is  in  reality 
even  more  wonderful.  It  is  only  very  feebly 
radioactive,  and  therefore  is  changing  excessively 
slowly,  but  it  changes,  we  believe,  into  radium, 


234  THE  ENERGY  IN  URANIUM 

expelling  several  a-particles  and  so  evolving  large 
amounts  of  energy  in  the  process.  Uranium  is  a 
heavier  element  than  radium,  and  the  relative 
weights  of  the  two  atoms,  which  is  a  measure  of 
their  complexity,  is  as  238  is  to  226.  This  bottle 
contains  about  a  pound  of  an  oxide  of  uranium 
which  contains  about  seven-eighths  of  its  weight  of 
the  element  uranium.  In  the  course  of  the  next 
few  thousand  million  years,  so  far  as  we  can  tell, 
it  will  change,  producing  over  thirteen  ounces  of 
radium,  and,  in  that  change  into  radium  alone, 
energy  is  given  out,  as  radioactive  energy,  aggre- 
gating of  itself  an  enormous  total,  while  the  radium 
produced  will  also  change,  giving  out  a  further 
enormous  aggregate  quantity  of  energy. 

So  that  uranium,  since  it  produces  radium,  con- 
tains   all    the    energy    contained    in   a  but  slightly 
smaller    quantity    of    radium    and    more.      It    may 
be   estimated    that    uranium    evolves    during   com- 
plete disintegration  some  thirteen  per  cent,   more 
energy  than   is   evolved  from  the  same  weight  of 
radium.      But  what  are  we  to  say  about  the  other 
heavy    elements  —  lead,     bismuth,     mercury,    gold, 
platinum,  etc. — although  their  atoms  are  not  quite 
so  heavy  as  uranium  or  radium,  and  although  none 
of  them,  so   far  as  we  yet  know,  are  disintegrating 
at  all  ?     Is  this  enormous  internal  store  of  energy 
confined    to    the    radioactive    elements,    that    is  to 


THE  INTERNAL  ENERGY  OF  ALL  MATTER      235 

the  few  which,  however  slowly,  are  actually  chang- 
ing ?  Not  at  all,  in  all  probability.  Regarded 
merely  as  chemical  elements  between  radioactive 
elements  and  non-radioactive  elements,  there  exists 
so  complete  a  parallelism  that  we  cannot  regard 
the  radioactive  elements  as  peculiar  in  possessing 
this  internal  store  of  energy,  but  only  as  peculiar 
in  evolving  it  at  a  perceptible  rate.  Radium 
especially  is  so  completely  analogous  in  its  whole 
chemical  nature,  and  even  in  the  character  of  its 
spectrum,  to  the  non-radioactive  elements,  barium, 
strontium,  and  calcium,  that  chemists  at  once 
placed  radium  in  the  same  family  as  these  latter, 
and  the  value  of  its  atomic  weight  confirms  the 
arrangement  in  the  manner  required  by  the 
Periodic  Law.  It  appears  rather  that  this  in- 
ternal store  of  energy  we  learned  of  for  the 
first  time  in  connection  with  radium  is  possessed 
to  greater  or  lesser  degree  by  all  elements  in 
common  and  is  part  and  parcel  of  their  internal 
structure. 

Let  us,  however,  for  the  sake  of  conciseness, 
leave  out  of  account  altogether  the  non-radioactive 
elements,  of  which  as  yet  we  know  nothing  cer- 
tainly. At  least  we  cannot  escape  from  the 
conclusion  that  the  particular  element  uranium 
has  relatively  more  energy  stored  up  within  it 


236  USEFUL  AND   USELESS  ENERGY 

even  than  radium.  Uranium  is  a  comparatively 
common  element.  The  world's  output  per  year  is 
to  be  reckoned  in  tens  of  tons. 

I  have  already  referred  to  the  total  amount  of 
energy  evolved  by  radium  during  the  course  of  its 
complete  change.  It  is  about  360,000  times  as  much 
energy  as  is  evolved  from  the  same  weight  of  coal  in 
burning.  The  energy  evolved  from  uranium  would 
be  some  thirteen  per  cent,  greater  than  from  the  same 
weight  of  radium.  This  bottle  contains  about  one 
pound  of  uranium  oxide,  and  therefore  about  fourteen 
ounces  of  uranium.  Its  value  is  about  £i.  Is  it 
not  wonderful  to  reflect  that  in  this  little  bottle  there 
lies  asleep  and  waiting  to  be  evolved  the  energy  of 
at  least  one  hundred  and  sixty  tons  of  coal  ?  The 
energy  in  a  ton  of  uranium  would  be  sufficient  to 
light  London  for  a  year.  The  store  of  energy  in 
uranium  would  be  worth  a  thousand  times  as  much 
as  the  uranium  itself,  if  only  it  were  under  our  control 
and  could  be  harnessed  to  do  the  world's  work  in  the 
same  way  as  the  energy  in  coal  has  been  harnessed 
and  controlled. 

There  is,  it  is  true,  plenty  of  energy  in  the 
world  which  is  practically  valueless.  The  energy 
of  the  tides  and  of  the  waste  heat  from  steam  fall 
into  this  category  as  useless  and  low-grade  energy. 
But  the  internal  energy  of  uranium  is  not  of  this 
kind.  The  difficulty  is  of  quite  another  character. 


THE  PROBLEM  OF  .TRANSMUTATION       237 

As  we  have  seen,  we  cannot  yet  artificially  ac- 
celerate or  influence  the  rate  of  disintegration  of 
an  element,  and  therefore  the  energy  in  uranium, 
which  requires  a  thousand  million  years  to  be 
evolved,  is  practically  valueless.  On  the  other 
hand,  to  increase  the  natural  rate,  and  to  break 
down  uranium  or  any  other  element  artificially, 
is  simply  transmutation.  If  we  could  accomplish 
the  one  so  we  could  the  other.  These  two  great 
problems,  at  once  the  oldest  and  the  newest  in 
science,  are  one.  Transmutation  of  the  elements 
carries  with  it  the  power  to  unlock  the  internal 
energy  of  matter,  and  the  unlocking  of  the  internal 
stores  of  energy  in  matter  would,  strangely  enough, 
be  infinitely  the  most  important  and  valuable  con- 
sequence of  transmutation. 

Let  us  consider  in  the  light  of  present  knowledge 
the  problem  of  transmutation,  and  see  what  the 
attempt  of  the  alchemist  involved.  To  build  up  an 
ounce  of  a  heavy  element  like  gold  from  a  lighter 
element  like  silver  would  require  in  all  probability 
the  expenditure  of  the  energy  of  some  hundreds  of 
tons  of  coal,  so  that  the  ounce  of  gold  would  be 
dearly  bought.  On  the  other  hand,  if  it  were 
possible  artificially  to  disintegrate  an  element  with 
a  heavier  atom  than  gold  and  produce  gold  from  it, 
so  great  an  amount  of  energy  would  probably  be 


238        THE  PROBLEM  OF  TRANSMUTATION 

evolved  that  the  gold  in  comparison  would  be  of 
little  account.  The  energy  would  be  far  more 
valuable  than  the  gold.  Although  we  are  as  ignor- 
ant as  ever  of  how  to  set  about  transmutation, 
it  cannot  be  denied  that  the  knowledge  recently 
gained  constitutes  a  very  great  help  towards  a 
proper  understanding  of  the  problem  and  its  ulti- 
mate accomplishment.  We  see  clearly  the  magnitude 
of  the  task  and  the  insufficiency  of  even  the  most 
powerful  of  the  means  at  our  disposal  in  a  way  not 
before  appreciated,  and  we  have  now  a  clear  per- 
ception of  the  tremendous  issues  at  stake.  Looking 
backwards  at  the  great  things  science  has  already 
accomplished,  and  at  the  steady  growth  in  power 
and  fruitfulness  of  scientific  method,  it  can  scarcely 
be  doubted  that  one  day  we  shall  come  to  break 
down  and  build  up  elements  in  the  laboratory  as 
we  now  break  down  and  build  up  compounds, 
and  the  pulses  of  the  world  will  then  throb  with 
a  new  source  of  strength  as  immeasurably  re- 
moved from  any  we  at  present  control  as  they  in 
turn  are  from  the  natural  resources  of  the  human 
savage. 

It  is,  indeed,  a  strange  situation  we  are  confronted 
with.  The  first  step  in  the  long,  upward  journey 
out  of  barbarism  to  civilisation  which  man  has  ac- 
complished appears  to  have  been  the  art  of  kindling 


THE  ART  OF  KINDLING  FIRE  239 

fire.  Those  savage  races  who  remain  ignorant  of 
this  art  are  regarded  as  on  the  very  lowest  plane. 
The  art  of  kindling  fire  is  the  first  step  towards  the 
control  and  utilisation  of  those  natural  stores  of 
energy  on  which  civilisation  even  now  absolutely 
depends.  Primitive  man  existed  entirely  on  the 
day-to-day  supply  of  sunlight  for  his  vital  energy, 
before  he  learned  how  to  kindle  fire  for  himself. 
One  can  imagine  before  this  occurred  that  he 
became  acquainted  with  fire  and  its  properties  from 
naturally  occurring  conflagrations. 

With  reference  to  the  newly  recognised  internal 
stores  of  energy  in  matter  we  stand  to-day  where 
primitive  man  first  stood  with  regard  to  the  energy 
liberated  by  fire.  We  are  aware  of  its  existence 
solely  from  the  naturally  occurring  manifestations 
in  radioactivity.  At  the  climax  of  that  civilisation 
the  first  step  of  which  was  taken  in  forgotten  ages 
by  primitive  man,  and  just  when  it  is  becoming 
apparent  that  its  ever-increasing  needs  cannot  in- 
definitely be  borne  by  the  existing  supplies  of 
energy,  possibilities  of  an  entirely  new  material 
civilisation  are  dawning  with  respect  to  which  we 
find  ourselves  still  on  the  lowest  plane— that  of  on- 
lookers with  no  power  to  interfere.  The  energy 
which  we  require  for  our  very  existence,  and  which 
Nature  supplies  us  with  but  grudgingly  and  in 
none  too  generous  measure  for  our  needs,  is  in 


24o      KEY  TO    THE  NEW  STORES  OF  ENERGY 

reality  locked  up  in  immense  stores  in  the  matter 
all  around  us,  but  the  power  to  control  and  use  it  is 
not  yet  ours.  What  sources  of  energy  we  can 
and  do  use  and  control,  we  now  regard  as  but  the 
merest  leavings  of  Nature's  primary  supplies.  The 
very  existence  of  the  latter  till  now  have  remained 
unknown  and  unsuspected.  When  we  have  learned 
how  to  transmute  the  elements  at  will  the  one  into 
the  other,  then,  and  not  till  then,  will  the  key  to  this 
hidden  treasure-house  of  Nature  be  in  our  hands. 
At  present  we  have  no  hint  of  how  even  to  begin 
the  quest. 

The  question  has  frequently  been  discussed 
whether  transmutation,  so  impossible  to  us,  is  not 
actually  going  on  under  the  transcendental  condi- 
tions obtaining  in  the  sun  and  the  stars.  We  have 
seen  that  it  is  actually  going  on  in  the  world  under 
our  eyes  in  a  few  special  cases  and  at  a  very  slow 
rate.  The  possibility  now  under  consideration,  how- 
ever, is  rather  that  it  may  be  going  on  universally 
or  at  least  much  more  generally,  and  at  a  much  more 
rapid  rate  under  celestial  than  under  terrestrial  con- 
ditions. From  the  new  point  of  view  it  may  be  said 
at  once  that  if  it  were  so,  many  of  the  difficulties 
previously  experienced  in  accounting  for  the  enor- 
mous and  incessant  dissipation  of  energy  throughout 
the  universe  would  disappear. 


WORLD-CREATING  INFLUENCES  241 

Last  century  has  wrought  a  great  change  in 
scientific  thought  as  to  the  nature  of  the  gigantic 
forces  which  have  moulded  the  world  to  its  present 
form  and  which  regulated  the  march  of  events 
throughout  the  universe.  At  one  time  it  was  cus- 
tomary to  regard  the  evolution  of  the  globe  as  the 
result  of  a  succession  in  the  past  times  of  mighty 
cataclysms  and  catastrophes  beside  which  the  erup- 
tions of  a  Krakatoa  or  Pelee  would  be  insignificant. 
Now,  however,  we  regard  the  main  process  of 
moulding  as  due  rather  to  ever-present,  continuous, 
and  irresistible  actions,  which,  though  operating 
so  slowly  that  over  short  periods  of  time  their  effect 
is  imperceptible,  yet  in  the  epochs  of  the  cosmical 
calendar  effected  changes  so  great  and  complete 
that  the  present  features  of  the  globe  are  but  a 
passing  incident  of  a  continually  shifting  scene. 
Into  the  arena  of  these  silent  world-creating  and 
destroying  influences  and  processes  has  entered  a 
new-comer — "  Radioactivity  "—and  it  has  not  re- 
quired long  before  it  has  come  to  be  recognised 
that  in  the  discovery  of  radioactivity,  or  rather  of 
the  sub-atomic  powers  and  processes  of  which  radio- 
activity is  merely  the  outward  and  visible  manifesta- 
tion, we  have  penetrated  one  of  Nature's  innermost 
secrets. 

Whether    or    no    the    processes    of    continuous 
atomic  disintegration    bulk  largely  in  the  scheme 

XX 


242     ULTIMATE  SOURCE   OF  COSMICAL  ENERGY 

of  cosmical  evolution,  at  least  it  cannot  be  gain- 
said that  these  processes  are  at  once  powerful 
enough  and  slow  enough  to  furnish  a  sufficient  and 
satisfactory  explanation  of  the  origin  of  those 
perennial  outpourings  of  energy  by  virtue  of  which 
the  universe  to-day  is  a  going  concern  rather  than 
a  cold,  lifeless  collocation  of  extinct  worlds.  Slow, 
irresistible,  incessant,  unalterable,  so  apparently 
feeble  that  it  has  been  reserved  to  the  genera- 
tion in  which  we  live  to  discover,  the  processes 
of  radioactivity,  when  translated  in  terms  of  a 
more  extended  scale  of  space  and  time,  appear 
already  as  though  they  well  may  be  the  ultimate 
controlling  factors  of  physical  evolution.  For  slow 
processes  of  this  kind  do  the  effective  work  of 
Nature,  and  the  occasional  intermittent  displays  of 
Plutonic  activity  correspond  merely  to  the  creaking 
now  and  again  of  an  otherwise  silent  mechanism 
that  never  stops. 

It  is  one  of  the  most  pleasing  features  of  this  new 
work  that  geologists  have  been  among  the  very 
first  to  recognise  the  applicability  and  importance 
of  it  in  their  science.  I  am  not  competent  to  deal 
adequately  with  or  discuss  the  geological  problems 
that  it  has  raised.  But  this  story  would  be  incom- 
plete if  I  did  not  refer,  though  it  must  be  but  briefly, 
to  the  labours  of  Professor  Strutt  who  initiated 


RADIUM  IN  ROCKS  243 

the  movement  and  to  those  of  Professor  Joly  who 
has  carried  it  on.  These  workers  carried  out 
careful  analyses  of  the  representative  rocks  in  the 
earth's  crust  for  the  amount  of  radium  they  con- 
tained. Absolutely,  the  quantity  of  radium  in  com- 
mon rocks  is  of  course  very  small,  although  with  the 
refined  methods  now  at  the  disposal  of  investigators 
it  is  quite  measurable.  The  important  fact  which 
has  transpired,  however,  is  that  the  rocks  examined 
contain  on  the  average  much  larger  quantities  of 
radium,  and  therefore  necessarily  of  its  original 
parent  uranium,  than  might  be  expected.  The 
amount  of  heat  which  finds  its  way  in  a  given  time 
from  the  interior  of  the  globe  to  the  surface  and 
thence  outwards  into  external  space  has  long  been 
accurately  known.  Strutt  concluded  that  if  there 
existed  only  a  comparatively  thin  crust  of  rocks  less 
than  fifty  miles  thick  of  the  same  composition,  as 
regards  the  content  of  radium,  as  the  average  of 
those  he  examined,  the  radium  in  them  would 
supply  the  whole  of  the  heat  lost  by  the  globe  to 
outer  space.  He  concluded  that  the  surface  rocks 
must  form  such  a  thin  crust,  and  that  the  interior 
of  the  globe  must  be  an  entirely  different  kind  of 
material,  free  from  the  presence  of  radium.  Other- 
wise the  world  would  be  much  hotter  inside  than  is 
known  to  be  the  case.  So  far  then  as  the  earth  is 
concerned,  a  quantity  of  radium  less  than  in  all 
probability  actually  exists  would  supply  all  the  heat 


244  RADIOACTIVITY 

lost  to  outer  space.  So  that  there  is  no  difficulty  in 
accounting  for  the  necessary  source  of  heat  to  main- 
tain the  existing  conditions  of  temperature  on  the 
earth  over  a  period  of  past  time  as  long  as  the 
uranium  which  produces  the  radium  lasts,  that  is  to 
say,  for  a  period  of  thousands  of  millions  of  years. 

Professor  Joly  in  his  interesting  work,  "  Radio- 
activity and  Geology,"  has  considered  in  detail  some 
of  the  consequences  of  the  existence  of  radioactive 
materials  in  the  earth.  One  of  the  specific  instances 
is  the  effect  of  the  radium  in  the  rocks  of  the  Simplon 
Tunnel  in  producing  the  unexpectedly  high  tempera- 
tures there  encountered.  From  a  radioactive  analysis 
of  these  rocks  he  came  to  the  conclusion  that  without 
undue  assumptions  it  is  possible  to  explain  the  differ- 
ences in  the  temperature  of  the  rocks  encountered  in 
boring  the  tunnel  by  the  differences  in  their  radium 
content. 

The  presence  in  the  rock  of  a  proportion 
amounting  to  a  few  million  millionths  of  radium 
above  the  normal  quantity  very  nearly  wrecked  the 
whole  enterprise.  From  the  importance  of  radio- 
activity in  this  instance,  of  a  tunnel  a  few  miles 
long  bored  through  a  mountain,  some  idea  may  be 
obtained  of  the  significance  of  the  new  discoveries 
in  the  general  problem  of  the  thermal  condition  of 
the  interior  of  the  globe.  Since  Strutt's  original 
work,  it  has  been  established  that  not  only  radium, 


AND   GBOLOGY  245 

but  all  the  other  radioactive  materials,  including  the 
whole  thorium  disintegration  series,  must  contribute 
an  important  quantity  of  heat,  so  that  his  estimate 
of  a  crust  only  fifty  miles  thick  is  in  reality  too 
great,  and  a  much  thinner  crust  would  suffice.  Joly 
has  had  the  courage  to  push  the  argument  to  its 
logical  conclusion,  and  has  supposed  that  the  radio- 
active materials  are  not  confined  to  a  thin  surface 
crust,  but  are  equally  distributed  throughout  the 
globe  in  much  the  same  proportions  as  they  are  in 
the  crust.  If  this  is  so,  there  is  no  escape  from  the 
conclusion  that  the  interior  of  the  earth,  so  far  from 
gradually  parting  with  its  heat  and  cooling  down, 
must  actually  be  getting  steadily  hotter.  The  heat 
generated  within,  even  after  the  lapse  of  hundreds 
of  millions  of  years,  would  scarcely  appreciably 
escape  from  the  surface,  for,  as  Lord  Kelvin  deduced, 
the  central  core  of  the  earth  must  be  almost  insulated 
thermally  from  the  surface,  owing  to  the  low  con- 
ductivity of  the  rocks  composing  the  crust.  He 
assumes  throughout  an  average  composition  of  the 
globe  of  two  parts  of  radium  per  million  million, 
which  is  considerably  below  the  average  he  found 
for  the  rocks  of  the  crust,  and  he  calculates  that  in 
the  course  of  a  hundred  million  years  this  minute 
quantity  will  produce  a  rise  of  the  temperature  of 
the  central  core  of  no  less  than  1,800°  C.  Unless, 
therefore,  this  heat  is  utilised  in  some  unknown 


246  THE   GEOLOGICAL  AND 

way,  or  the  disintegration  of  the  radio-elements  is 
prevented  by  the  high  temperature  and  pressure, 
the  ultimate  fate  of  the  globe  must  be  very  much  as 
depicted  in  the  Biblical  tradition.  Sooner  or  later 
the  crust  must  succumb  to  the  ever  increasing 
pressure  within,  and  the  earth  must  become  again, 
what  it  is  supposed  once  to  have  been,  a  vastly 
swollen  globe  of  incandescent  gas.  As  Joly  remarks, 
there  is  no  evidence  that  this  has  not  already 
occurred  more  than  once,  nor  assurance  that  it  will 
not  recur.  So  far  as  physical  science  yet  can  deduce, 
the  accumulation  of  thermal  energy  within  a  world 
containing  elements  undergoing  atomic  disintegra- 
tion during  the  " geological  age"  must  alternate 
with  a  state  of  things  which  might  be  termed  "the 
incandescent  age,"  in  which  this  accumulated  energy 
is  dissipated  by  radiation.  This  periodic  cycle  of 
changes  must  continue  until  the  elements  in  question 
have  disintegrated — that  is,  over  a  period  which 
radioactive  measurements  indicate  is  of  the  order 
of  tens  or  hundreds  of  thousands  of  millions  of  years, 
During  the  incandescent  age  the  loss  of  heat  by 
radiation,  which  increases  according  to  the  fourth 
power  of  the  temperature,  is  immensely  greater  than 
could  be  supplied  even  by  atomic  disintegration. 

Thus,  if  the  known  laws  hold,  it  is  certain  that  the 
present  loss  of  heat  of  the  sun  cannot  be  supplied  by 
the  presence  of  radium.  For  this  to  be  the  case  a 
very  large  part  of  the  sun's  mass  must  consist  of 


THE  INCANDESCENT  AGES  247 

uranium,  and  this  we  know  from  the  spectroscope 
is  very  improbable.  Still,  it  is  by  no  means  to 
be  concluded  that  the  heat  of  the  sun  and  stars  is 
not  in  the  first  place  of  internal  rather  than,  as  has 
been  the  custom  to  regard  it,  of  external  origin. 

As  soon  as  sufficient  of  the  heat  energy  of  the 
sun  has  been  radiated  away  for  a  solid  crust  to  form, 
the  poor  thermal  conductivity  of  this  crust  at  once 
reduces  the  radiation  loss  to  a  negligible  figure 
again,  a  fresh  geological  age  is  inaugurated,  and 
again  the  heat  accumulates  within.  This  view,  that 
the  elements  contain  within  themselves  the  energy 
from  which  Nature  obtains  her  primary  supplies, 
and  that  in  cosmical  time  "  geological  age "  and 
"  incandescent  age  "  alternate  as  the  night  and  day, 
however  imperfect  it  may  still  be,  is  at  least  more 
in  harmony  with  existing  knowledge  than  the  older 
conventional  view  that  the  universe  was  wound  up 
once  for  all  in  the  beginning  like  a  clock  to  go  for 
a  certain  time,  for  the  most  part  quietly  and  unevent- 
fully, pursuing  its  allotted  path  towards  ultimate 
physical  stagnation  and  death.  But  what  a  picture 
it  conjures  up  of  life  and  of  the  precariousness  of 
its  tenure — from  its  lowest  beginnings  to  its  highest 
evolution,  not  a  permanent  accomplishment,  but 
a  process  to  be  inaugurated  and  consummated  afresh, 
if  at  all,  between  the  ending  and  beginning  of  each 
new  cosmical  day ! 


248  COSMIC AL  EVOLUTION 

To  escape  from  this  conclusion  it  is  necessary  to 
suppose  that  atomic  disintegration  is  cosmically  not 
the  inevitable  uncontrollable  process  it  has  hitherto 
been  proved  to  be  under  all  laboratory  conditions, 
but  that  under  conditions  of  pressure  and  tempera- 
ture, such  as  exist  in  the  interior  of  a  world,  it  may 
either  be  stopped  altogether,  or  compensated  for  by 
unknown  complementary  processes  of  atomic  syn- 
thesis in  which  energy  is  taken  up. 

Be  that  as  it  may,  our  outlook  on  the  physical 
universe  has  been  permanently  altered.  We  are  no 
longer  the  inhabitants  of  a  universe  slowly  dying 
from  the  physical  exhaustion  of  its  energy,  but  of 
a  universe  which  has  in  the  internal  energy  of  its 
material  components  the  means  to  rejuvenate  itself 
perennially  over  immense  periods  of  time,  inter- 
mittently and  catastrophically,  which  is  the  first 
possibility  that  presents  itself,  or  continuously  and 
in  orderly  fashion,  if  there  exist  compensating 
phenomena  still  outside  the  ken  of  science. 

The  world  probably  being  of  much  greater  antiquity 
than  physical  science  has  thought  to  be  possible,  it 
is  interesting  and  harmless  to  speculate  whether  man 
has  shared  with  the  world  its  more  remote  history. 

In  this  connection  it  is  curious  how  strangely  some 
of  the  old  myths  and  legends  about  matter  and 
man  appear  in  the  light  of  the  recent  knowledge. 


OLD  LEGENDS,  RECALLED  249 

Consider,  for  example,  the  ancient  mystic  symbol  of 
matter,  known  as  Ouroboros — "  the  tail  devourer  "- 
which  was  a  serpent,  coiled  into  a  circle  with  the 
head  devouring  the  tail,  and  bearing  the  central 
motto,  "  The  whole  is  one."  This  symbolises 
evolution  ;  moreover,  it  is  evolution  of  matter — the 
very  latest  aspect  of  evolution — the  existence  of 
which  was  strenuously  denied  by  Clerk  Maxwell  and 
others  of  only  last  century.  The  idea  which  arises 
in  one's  mind  as  the  most  attractive  and  consistent 
explanation  of  the  universe  in  the  light  of  present 
knowledge  is,  perhaps,  that  matter  is  breaking  down 
and  its  energy  being  evolved  and  degraded  in  one 
part  of  a  cycle  of  evolution,  and  in  another  part,  still 
unknown  to  us,  the  matter  is  being  again  built  up 
with  the  utilisation  of  the  waste  energy  If  one 
wished  to  symbolise  such  an  idea,  in  what  better 
way  could  it  be  done  than  by  the  ancient  tail- 
devouring  serpent  ? 

Some  of  the  beliefs  and  legends  which  have 
come  down  to  us  from  antiquity  are  so  universal 
and  deep-rooted  that  we  are  accustomed  to  consider 
them  almost  as  old  as  the  race  itself.  One  is 
tempted  to  inquire  how  far  the  unsuspected  apt- 
ness of  some  of  these  beliefs  and  sayings  to  the 
point  of  view  so  recently  disclosed  is  the  result  of 
mere  chance  or  coincidence,  and  how  far  it  may  be 
evidence  of  a  wholly  unknown  and  unsuspected 


250  THE  ELIXIR   OF  LIFE 

ancient  civilisation  of  which  all  other  relic  has  dis- 
appeared. It  is  curious  to  reflect,  for  example, 
upon  the  remarkable  legend  of  the  philosopher's 
stone,  one  of  the  oldest  and  most  universal  beliefs, 
the  origin  of  which,  however  far  back  we  penetrate 
into  the  records  of  the  past,  we  do  not  probably 
trace  to  its  real  source.  The  philosopher's  stone 
was  accredited  the  power  not  only  of  transmuting 
the  metals,  but  of  acting  as  the  elixir  of  life.  Now, 
whatever  the  origin  of  this  apparently  meaningless 
jumble  of  ideas  may  have  been,  it  is  really  a  perfect 
and  but  very  slightly  allegorical  expression  of  the 
actual  present  views  we  hold  to-day.  It  does  not 
require  much  effort  of  the  imagination  to  see  in 
energy  the  life  of  the  physical  universe,  and  the 
key  to  the  primary  fountains  of  the  physical  life 
of  the  universe  to-day  is  known  to  be  transmuta- 
tion. Is  then  this  old  association  of  the  power 
of  transmutation  with  the  elixir  of  life  merely  a 
coincidence  ?  I  prefer  to  believe  it  may  be  an  echo 
from  one  of  many  previous  epochs  in  the  unre- 
corded history  of  the  world,  of  an  age  of  men 
which  have  trod  before  the  road  we  -are  treading 
to-day,  in  a  past  possibly  so  remote  that  even  the 
very  atoms  of  its  civilisation  literally  have  had  time 
to  disintegrate. 

Let  us  give  the  imagination  a  moment's  further 
free  scope  in  this  direction,  however,  before  closing. 


THE  FALL  AND  ASCENT  OF  MAN          251 

What  if  this  point  of  view  that  has  now  suggested 
itself  is  true,  and  we  may  trust  ourselves  to  the 
slender  foundation  afforded  by  the  traditions  and 
superstitions  which  have  been  handed  down  to  us 
from  a  prehistoric  time  ?  Can  we  not  read  into 
them  some  justification  for  the  belief  that  some 
former  forgotten  race  of  men  attained  not  only  to 
the  knowledge  we  have  so  recently  won,  but  also 
to  the  power  that  is  not  yet  ours  ?  Science  has 
reconstructed  the  story  of  the  past  as  one  of  a  con- 
tinuous Ascent  of  Man  to  the  present-clay  level  of 
his  powers.  In  face  of  the  circumstantial  evidence 
existing  of  this  steady  upward  progress  of  the  race, 
the  traditional  view  of  the  Fall  of  Man  from  a 
higher  former  state  has  come  to  be  more  and  more 
difficult  to  understand.  From  our  new  standpoint 
the  two  points  of  view  are  by  no  means  so  irrecon- 
cilable as  they  appeared.  A  race  which  could 
transmute  matter  would  have  little  need  to  earn  its 
bread  by  the  sweat  of  its  brow.  If  we  can  judge 
from  what  our  engineers  accomplish  with  their 
comparatively  restricted  supplies  of  energy,  such 
a  race  could  transform  a  desert  continent,  thaw  the 
frozen  poles,  and  make  the  whole  world  one  smiling 
Garden  of  Eden.  Possibly  they  could  explore  the 
outer  realms  of  space,  emigrating  to  more  favour- 
able worlds  as  the  superfluous  to-day  emigrate  to 
more  favourable  continents.  The  legend  of  the 


252  THE  NEW  PROSPECT 

Fall  of  Man,  possibly,  may  be  all  that  has  survived 
of  such  a  time  before,  for  some  unknown  reason, 
the  whole  world  was  plunged  back  again  under  the 
undisputed  sway  of  Nature,  to  begin  once  more  its 
upward  toilsome  journey  through  the  ages. 

The   vistas  of  new  thought  which  have  opened 
out  in  all  directions  in  the  physical  sciences,  to  which 
man  is  merely  incidental  and  external,  must  in  turn 
react  powerfully  upon  those  departments  of  thought 
in  which  man  is  central  and  supreme.     We  find  our- 
selves in  consequence  of  the  progress  of  physical 
science  at  the  pinnacle  of  one  ascent  of  civilisation, 
taking  the  first  step  upwards  out  on  to  the  lowest 
plane  of  the  next.     Above  us  still  rises  indefinitely 
the  ascent  to  physical  power — far  beyond  the  dreams 
of  mortals  in  any  previous   system  of  philosophy. 
These  possibilities  of  a  newer  order  of  things,  of  a 
more  exalted  material  destiny  than  any  which  have 
been  foretold,  are  not  the  promise  of  another  world. 
They  exist  in  this,  to  be  fought  and  struggled  for  in 
the  old  familiar  way,  to  be  wrung  from  the  grip  of 
Nature,  as    all    our   achievements    and    civilisation 
have,  in  the  past,  been  wrung  by  the  labour  of  the 
collective  brain  of  mankind  guiding,  directing,  and 
multiplying  the  individual's   puny  power.     This  is 
the    message  of  hope  and   inspiration  to  the  race 
which  radium  has  contributed  to  the  great  problems 
of  existence.     No   attempt  at  presentation  of  this 


THE  NEW  PROSPECT  253 

new  subject  could  be  considered  complete  which  did 
not,  however  imperfectly,  suggest  something  of  this 
side.  It  is  fitting  to  attempt  to  see  how  far  purely 
physical  considerations  will  take  us  in  delimiting 
the  major  controlling  influences  which  regulate  our 
existence. 

Surveying  the  long  chequered,  but  on  the  whole 
continuous,  ascent  of  man  from  primeval  conditions 
to  the  summit  of  his  present-day  powers,  what  has 
it  all  been  at  bottom  but  a  fight  with  Nature  for 
energy — for  that  ordinary  physical  energy  of  which 
we  have  said  so  much  ?  Physical  science  sums  up 
accurately  in  that  one  generalisation  the  most  funda- 
mental aspect  of  life  in  the  sense  already  defined. 

Of  course  life  depends  also  on  a  continual  supply 
of  matter  as  well  as  on  a  continual  supply  of  energy, 
but  the  struggle  for  physical  energy  is  probably  the 
more  fundamental  and  general  aspect  of  existence 
in  all  its  forms.  The  same  matter,  the  same  chemical 
elements,  serve  the  purposes  of  life  over  and  over 
again,  but  the  supply  of  fresh  energy  must  be  con- 
tinuous. By  the  law  of  the  availability  of  energy, 
which,  whether  universal  or  not,  applies  universally 
within  our  own  experience,  the  transformations  of 
energy  which  occur  in  Nature  are  invariably  in 
the  one  direction,  the  more  available  forms  pass- 
ing into  the  waste  and  useless  unavailable  kind,  and 
this  process,  so  far  as  we  yet  know,  is  never  reversed. 


254  THE  NEW  PROSPECT 

The  same  energy  is  available  but  once  The 
struggle  for  existence  is  at  the  bottom  a  continuous 
struggle  for  fresh  physical  energy. 

This  is  as  far  as  the  knowledge  available  last 
century  went.  What  is  now  the  case  ?  The 
aboriginal  savage,  ignorant  of  agriculture  and  of  the 
means  of  kindling  fire,  perished  from  cold  and 
hunger  unless  he  subsisted  as  a  beast  of  prey 
and  succeeded  in  plundering  and  devouring  other 
animals.  Although  the  potentialities  of  warmth 
and  food  existed  all  round  him,  and  must  have  been 
known  to  him  from  natural  processes,  he  knew  not 
yet  how  to  use  them  for  his  own  purposes.  It  is 
much  the  same  to-day.  With  all  our  civilisation, 
we  still  subsist,  struggling  among  ourselves  for  a 
sufficiency  of  the  limited  supply  of  physical  energy 
available,  while  all  around  are  vast  potentialities  of 
the  means  of  sustenance,  we  know  of  from  naturally 
occurring  processes,  but  do  not  yet  know  how  to  use 
or  control.  Radium  has  taught  us  that  there  is  no 
limit  to  the  amount  of  energy  in  the  world  available 
to  support  life,  save  only  the  limit  imposed  by  the 
boundaries  of  knowledge. 

It  cannot  be  denied  that,  so  far  as  the  future 
is  concerned,  an  entirely  new  prospect  has  been 
opened  up.  By  these  achievements  of  experimental 
science  Man's  inheritance  has  increased,  his  aspi- 
rations have  been  uplifted,  and  his  destiny  has 


THE  NEW  PROSPECT  255 

been  ennobled  to  an  extent  beyond  our  present 
power  to  foretell.  The  real  wealth  of  the  world  is 
its  energy,  and  by  these  discoveries  it,  for  the  first 
time,  transpires  that  the  hard  struggle  for  existence 
on  the  bare  leavings  of  natural  energy  in  which  the 
race  has  evolved  is  no  longer  the  only  possible  or 
enduring  lot  of  Man.  It  is  a  legitimate  aspira- 
tion to  believe  that  one  day  he  will  attain  the 
power  to  regulate  for  his  own  purposes  the  primary 
fountains  of  energy  which  Nature  now  so  jealously 
conserves  for  the  future.  The  fulfilment  of  this 
aspiration  is,  no  doubt,  far  off,  but  the  possibility 
alters  somewhat  the  relation  of  Man  to  his  environ- 
ment, and  adds  a  dignity  of  its  own  to  the  actuali- 
ties of  existence. 


CHAPTER  XII. 

The  thorium  disintegration  series — Source  and  radioactivity  of 
thorium  compounds — Mesothorium  and  radiothorium — Variation 
of  radioactivity  of  thorium  compounds  with  time  —  Analogy 
between  the  thorium  and  uranium  series — Mesothorium  and  its 
changes  of  activity  with  time — Its  chemical  resemblance  to  radium 
— Abundance  of  the  raw  material  of  mesothorium — The  thorium 
emanation — Experiments  with  radiothor  um — Thorium  A — The 
actinium  disintegration  series — The  question  of  the  origin  of 
actinium — Its  great  scarcity — Multiple  atomic  disintegration — The 
actinium  emanation — Actinium  A — The  unsolved  riddle  of  matter, 

r  I  ^HOSE  who  have  mastered  the  intricacies  of 
-L  the  uranium  disintegration  series  may  wish 
to  know  something  of  the  other  two  great  dis- 
integration series  known  to  science,  the  thorium 
and  the  actinium  series.  The  thorium  disintegra- 
tion series  is  becoming  increasingly  important, 
and  its  consideration  does  not  involve  any  new 
principles.  Thorium  is  an  element  which  was  at 
one  time  rare  and  little  known  even  to  chemists,  but 
has  come  into  prominence  during  the  last  twenty 
years,  because  oi'  its  use  as  a  constituent  of  the 
incandescent  gas-mantle,  which  is  composed  of  about 

99  per  cent,  of  thorium  oxide,  and   i   per  cent,  of 

256 


MONAZITE  SAND  257 

cerium  oxide.  Fairly  abundant  sources  of  thorium 
have  been  discovered  in  the  sands  of  certain  coasts 
in  Brazil,  North  and  South  Carolina,  India,  etc., 
where  a  natural  concentration  has  taken  place  by 
the  action  of  the  sea- waves  of  the  particles  of  the 
heavy  mineral  monazite,  which  occurs  as  a  minute 
constituent  in  many  rocks,  and  in  the  sands  derived 
from  them  by  the  action  of  weathering  agencies. 
Monazite  sand  usually  contains  about  4  per  cent,  of 
thorium  oxide,  and  from  it  every  year  hundreds  of 
tons  of  pure  thorium  salts  are  now  manufactured  for 
the  gas-mantle  industry.  As  already  described,  the 
usual  ^-radioactivity  of  commercial  thorium  com- 
pounds is  of  about  the  same  strength  as  that  of 
pure  uranium  compounds,  but  the  13 -  and  7-,  or 
penetrating  activity,  is  several  times  less  intense. 
We  have  seen  (p.  212)  that  in  the  uranium  minerals, 
although  several  intermediate  products  of  the  dis- 
integration of  uranium  are  present,  with  periods  of 
life  sufficiently  long,  and  radioactivity  sufficiently 
important,  to  repay  extraction,  it  is  practicable  to 
extract  only  one  of  these — namely,  radium.  In  the 
thorium  minerals  there  are  two  such  products,  named 
mesothorium  and  radiothorium,  and  though  their 
periods  of  average  life,  about  eight  years  and  three 
years  respectively,  are  very  much  less  than  that  of 
radium,  they  are  sufficiently  long  to  make  their  extrac- 
tion and  utilisation  practicable.  Whereas  the  sources 
s 


258  THE  IMPORTANCE  OF 

of  radium  are  costly  and  comparatively  limited  in 
amount,  the  by-products  of  the  thorium  industry, 
after  the  extraction  of  the  technically  valuable 
thorium,  are  the  source  from  which  mesothorium 
and  radiothorium  are  extracted.  Much  greater 
quantities  of  these  by-products  have  to  be  handled, 
it  is  true,  than  in  the  extraction  even  of  radium  from 
pitchblende  to  produce  similar  results.  The  new 
substances  must,  on  this  account,  always  be  some- 
what costly  to  produce.  But  in  the  by-products  of 
a  single  year's  manufacture  of  thorium  the  new 
products  capable  of  being  extracted  possess  as 
much  radioactivity  as  at  least  an  ounce  of  pure 
radium.  They  thus  offer  an  abundant  source  of 
radioactive  material,  which  at  present  is  mostly 
wasted,  and  the  product,  while  it  lasts,  is  in  every 
respect  the  equal  of  radium  in  its  properties.  The 
only  disadvantages  it  possesses  is  its  relatively 
much  shorter  period  of  life. 

The  discoveries  in  the  thorium  series  of  these 
two  technically  valuable  members  were  made  by 
Ott.o  Hahn,  who  has  worked  both  with  Sir 
William  Ramsay  and  Professor  Rutherford,  com- 
paratively recently,  after  the  rest  of  the  members 
had  become  quite  well  known.  The  historical 
development  of  the  subject  from  the  first  discovery 
of  the  radioactivity  of  thorium  compounds  up  to  the 
present  time  is  a  most  interesting  chapter  to  the 


THE   THORIUM  SERIES  259 

student,  but  would  unduly  complicate  the  subject  if 
considered  here.  It  is  better  to  proceed  in  order 
through  the  thorium  disintegration  series  as  it  is 
at  present  known,  apart  from  historical  considera- 
tions as  to  the  order  in  which  they  were  discovered, 
though  as  a  matter  of  fact,  the  first  members  were 
the  last  discovered. 

Unlike  pure  uranium  salts,  which,  a  few  months 
after  preparation,  have  a  definite  constant  radio- 
activity, consisting  of  all  three  types  of  rays,  the 
a-activity  being  due  to  uranium,  and  the  /3-  and  7- 
activity  to  the  short-lived  uranium  X  in  equilibrium 
with  it,  thorium  salts,  though  chemically  pure,  vary 
continuously  in  their  whole  radioactivity  for  twenty 
or  thirty  years  after  manufacture.  Even  after  these 
periods,  slight  changes  must  still  be  going  on,  and 
probably  fifty  years  would  have  to  elapse  before 
they  became  quite  inappreciable.  But  in  spite  of 
the  great  apparent  differences  between  the  two 
elements,  there  is  a  real  close  analogy  in  their 
disintegration  series,  every  one  of  the  eleven  known 
members  of  the  thorium  series  having  an  analogue 
in  the  eleven  members  of  the  uranium  series  as  far 
as  radium  D,  at  which  point  the  thorium  disintegra- 
tion appears  to  come  to  an  end.  These  analogous 
members  in  the  two  series  usually  give  out  similar 
kinds  of  rays,  and  although  their  periods  are  often 
widely  different,  there  is  a  rough  correspondence  in 


260  THE    URANIUM  AND 

the  two  series  between  the  relative  periods  of  the 
successive  members,  the  periods  in  the  thorium 
series  being,  however,  usually  much  less  than  in  the 
uranium  series.  Thus  uranium  I,  with  its  period  of 
8,ooo,oooyyears,  gives  a-rays,  and  is  followed  by 
uranium  II,  also  giving  a-rays,  of  estimated  period  of 
a  few  million  years.  This  is  followed  by  uranium  X, 
giving  0-rays,  of  period  35*5  days.  This  produces 
ionium,  which  gives  a-rays,  and  has  the  estimated 
period  of  200,000  years.  Ionium,  in  turn,  produces 
radium,  which  gives  a-rays,  and  has  a  period  of  2,500 
years.  Thorium  itself  is  provisionally  estimated  to 
have  a  period  about  three  times  longer  than  uranium  I , 
and  gives  only  a-rays.  It  produces  by  its  disintegra- 
tion "mesothorium  I,"  which  does  not  give  any  im- 
portant rays,  and  has  a  period  of  7*9  years.  It  corre- 
sponds with  uranium  II,  except  that  no  a-rays  are 
expelled.  Its  product  is  called  "mesothorium  II," 
which  corresponds  very  closely  with  uranium  X, 
giving  out  powerful  /3-  and  7- rays,  and  having  a 
period  of  only  8*9  hours.  It  produces  in  turn 
"  radiothorium,"  which  corresponds  perfectly  with 
ionium,  giving  a-rays,  and  having  a  period  of  2*9 
years.  These  last  two  substances  are  chemically 
identical  with  one  another,  and  also  with  thorium 
itself,  and  cannot  be  separated  by  any  known  method 
when  mixed  together.  This  fact  is  of  considerable 
importance,  as  thorium  when  separated  from  a 


THORIUM  SERIES  COMPARED  261 

mineral,  always  contains  at  first  all  the  radiothorium 
in  the  mineral  and  also  all  the   ionium,  if  uranium 
was  also  present,  as  is  almost  invariably  the  case. 
The  product  of  radiothorium  is  thorium  X,  which 
corresponds   with   radium,   giving  a-rays,   but   hav- 
ing   a   period    of  only    5*6    days.     Thorium    X    is 
chemically    identical    with    radium,    and    also    with 
mesothorium   I.     This  chemical  identity  of  radium 
and  mesothorium   I   is  the   dominating  fact   in  the 
separation  of  these  new  substances,  as  will  later  be 
more  clear.     After  thorium  X,  the  thorium  emana- 
tion   results,  corresponding    perfectly    in    its  whole 
nature  as  a  member  of  the  argon  family  of  gases, 
with  the  radium  emanation,  giving  a-rays,  but  having 
the  much  shorter  period  of  only  76  seconds.      Its 
product    is    the    thorium    active    deposit,    of  which 
the  first  three  members,    called  thorium  A,  B,  C, 
are  almost  precisely  analogous  to  the  corresponding 
radium  members,  except  in  period.     The  period  of 
thorium  A  is  only  one-fifth  of  a  second.     Those  of 
thorium  B  and  C  are   15*3  hours  and  79  minutes 
respectively.     These  last  are  the  only  two,  except 
thorium  itself,  possessing  periods  longer  than  the 
corresponding    members    of    the    uranium    series. 
Lastly,   there  exists,  as  the  product  of  thorium  C, 
thorium  D,  the  last  active   member  known,  which 
gives  ft-  and  7-rays,  and  has  the  short  period  of  4*5 
minutes.      It  has  little  analogy  to  radium  D.     The 


262 


MESOTHORIUM 


ultimate  product  of  thorium  is  quite  unknown.  All 
that  can  yet  be  said  is  that  its  atomic  weight, 
calculated  from  that  of  thorium  and  the  number  of 
a-particles  expelled,  is  208,  and  this  is  the  atomic 
weight  of  bismuth !  This  is  one  of  the  problems 
time  alone  will  settle,  but  it  should  be  settled  in  a 
very  few  years.  The  complete  thorium  disintegra- 
tion series  is  shown  below  : 


Thorium.          Meso-  Meso- 

ihorium  I.  thorium  II. 

25,000,000,000        7'9  8*9 

(?)  years.            years.  hours. 


Radio-  Thorium 

thorium.  X. 

2-91  5*35 

years.  days. 


Emanation. 


76 

seconds. 


Thorium 
A. 

0-2 

second. 


Thorium 
B. 


hours. 


Thorium 
C. 

.79 
minutes. 


Thorium  Thorium 

D.  E. 

4*5  (unknown.) 

minutes. 


FIG.   32. 


It  is  clear  that  mesothorium  I,  with  the  period  of 
average  life  of  nearly  eight  years,  being  both  the 
first  and  the  longest  lived  of  the  successive  products, 
is  the  centre  of  interest.  The  radioactivity  of  the 
element  thorium  itself,  consisting  only  of  low-range 
a-rays,  of  relatively  feeble  intensity  because  of  the 
enormous  period  of  the  element,  is  technically  and 
scientifically  even  of  less  interest  than  that  of  uranium. 


MESOTHORIUM  263 

Mesothorium,  however,  corresponds  to  radium  in  that 
it  can  be  concentrated,  and  the  greater  part  of  the 
radioactivity  of  many  tons  of  minerals  can  be  separated 
in  a  preparation  weighing  less  than  a  gram.  Just  as 
when  radium  is  first  prepared  it  gives  only  the  rela- 
tively unimportant  a-activity  proper  to  itself,  but  in 
course  of  time  develops  enormously  in  all  its  activity 
due  to  the  growth  and  accumulation  of  its  products, 
so  it  is  with  mesothorium.  Freshly  prepared  and 
free  from  its  products,  it  has  practically  no  activity. 
In  the  course  of  a  few  hours  the  strong  penetrating 
activity  of  its  short-lived  product,  mesothorium  II, 
develops,  and  in  two  or  three  days  this  reaches  a 
maximum  or  equilibrium  value.  This  part  of  the 
activity  then  remains,  so  long  as  the  preparation  is 
not  chemically  treated,  apparently  constant,  but 
actually  decaying  very  slowly.  This  decay  is  to 
half  the  initial  value  after  5-5  years,  to  a  quarter 
after  1 1  years,  and  so  on.  But  the  product  of  this 
change  is  radiothorium,  which  gives  a-rays  ;  and,  just 
as  in  the  case  of  radium,  this  is  followed  by  a  small 
host  of  short-lived  products,  some  of  which  give  a. 
and  others  ft-  and  7-rays.  What  actually  happens, 
therefore,  is  that  in  addition  to  the  rapid  growth  of 
ft-  and  y-rays  already  discussed,  a  slow,  steady 
increase  of  the  a-,  /3-,  and  y-activity  of  a  mesothorium 
preparation  takes  place  for  many  years  after  its  pre- 
paration, due  to  the  growth  and  accumulation  of 


264  RADIOTHORTUM  AND 

radiothorium  and  its  products.  It  is  calculated  that 
this  increase  will  go  on  for  about  four  and  a  half 
years,  and  then  the  activity  of  the  preparation  will 
reach  a  maximum,  the  penetrating  activity  (/5-  and 
7-rays)  being  then  nearly  twice  that  at  two  days  after 
preparation.  From  then  onwards  the  regular  slow 
decay  of  all  the  radioactivity  will  set  in,  and  continue 
with  the  half-period  of  five  and  a  half  years,  as  already 
considered.  Twenty  years  after  preparation  the 
activity  will  be  some  12  per  cent,  whilst  after  a 
century  it  would  be  less  than  i,oooth  per  cent,  of 
the  maximum  activity. 

In  practice,  however,  the  change  is  even  more 
complicated  than  this  on  account  of  the  invariable 
presence  of  radium  in  the  mesothorium  preparations. 
Radium  and  mesothorium  form,  as  already  remarked, 
an  example  of  which  now  so  many  exist  in  radio- 
activity, of  two  different  elements,  having  entirely 
different  radioactive,  but  entirely  identical,  chemical 
character.  For  a  long  time  the  nature  of  the 
chemical  processes  used  to  extract  mesothorium  from 
the  by-products  of  monazite  was  kept  secret.  It  was 
thought  that  they  were  peculiarly  difficult  and  for- 
bidding. I  was  therefore  surprised  and  interested 
to  find — and  the  same  discovery  was  made  at  about 
the  same  time  by  Professor  Marckwald  in  Berlin — 
that  mesothorium  and  radium  behaved  in  chemical 
processes  identically.  In  consequence  the  extraction 


MESOTHORIUM  265 

of  mesothorium  from  monazite  residues  is  entirely 
similar  in  principle  to  that  of  radium  from  pitch- 
blende residues.  Since  monazite  always  contains  a 
minute  amount  of  uranium,  and  therefore  the  corre- 
sponding quantity  of  radium,  the  mesothorium 
separated  always  contains  the  radium  also.  No  suc- 
cessful separation  has  as  yet  been  achieved.  After  a 
lengthy  fractional  crystallisation  of  the  mixture  I 
found  the  relative  proportions  of  the  two  elements 
entirely  unaltered.  Technical  mesothorium  owes 
about  12  per  cent,  of  what  has  been  termed  its 
maximum  activity  (that  after  four  and  a  half  years)  to 
radium.  This  activity  will  remain  when  all  that  due 
to  mesothorium  has  completely  decayed  away.  In 
practice,  therefore,  the  decay  of  the  preparations  will 
be  appreciably  slower  than  if  radium  were  absent. 

These  discoveries  have  thus  resulted  in  the  pro- 
vision of  an  effective  substitute  for  radium,  which  for 
such  purposes  as  medical  application,  or  for  general 
researches  in  the  properties  of  the  new  radiations, 
are,  while  the  activity  lasts,  its  equal  in  every  respect. 
Indeed,  it  is  possible  to  obtain  mesothorium  pre- 
parations many  times  more  concentrated  in  their 
activity  than  pure  radium  salts.  There  is  no  dearth 
of  the  raw  material,  which  hitherto  has  been  a  wasted 
product.  A  single  year's  production  would  suffice  to 
produce  a  preparation  having  many  times  the  activity 
of  all  the  radium  ever  extracted  in  the  whole  world. 


266  THE  EMANATION 

But  of  course,  from  the  strictly  scientific  point  of 
view,  the  radioactivity  of  mesothorium  is  as  distinct 
from  that  of  radium  as  copper  is  from  zinc,  or  as  one 
flower  is  from  another.  It  will  be  of  interest  to  con- 
centrate upon  some  of  the  chief  resemblances  and 
differences  in  the  two  disintegration  series. 

The  thorium  emanation  was  the  first  of  the  three 
emanations  to  be  discovered,  and  had  been  fairly 
completely  investigated  by  Rutherford  before  the 
others  were  known.  It  is  given  off  in  greater  or  less 
degree  by  all  thorium  compounds.  If  the  radio- 
activity of  the  compound  is  measured  by  placing  it 
in  a  closed  electroscope,  the  activity  is  found  to 
increase  for  about  ten  minutes,  owing  to  the  accumu- 
lation of  the  emanation,  and  then  remains  constant  if 
the  instrument  is  not  disturbed.  But  if  a  current  of 
air  is  blown  through  the  instrument,  it  sweeps  out  the 
emanation,  and  the  activity  is  correspondingly  re- 
duced. On  stopping  the  blast  of  air,  it  rises  again 
precisely  as  at  first.  Uranium  compounds  show  no 
trace  of  this  behaviour,  as  they  do  not  generate  an 
emanation.  The  products  of  the  disintegration  of  the 
thorium  emanation  are  known  as  the  thorium  active 
deposit,  and  they  manifest  themselves  in  much  the 
same  way  as  the  radium  active  deposit,  being  attracted 
to  the  negatively  charged  surface  in  an  electric  field. 
They  last  much  longer,  however,  the  period  of  half- 
decay  being  about  eleven  hours  instead  of  about 


OF  THORIUM  267 

half  an  hour,  and,  in  consequence,  they  take  longer 
to  accumulate.  In  a  vessel  containing  a  thorium 
or,  better,  a  radiothorium  preparation,  which  acts  as 
a  constant  source  of  the  evanescent  thorium  emana- 
tion, the  active  deposit  on  the  walls  of  the  vessel  (or 
on  the  negative  electrode,  if  an  electric  field  is  used), 
goes  on  increasing  in  amount  for  about  two  days, 
whereas  in  the  radium  emanation  the  active  deposit 
reaches  the  maximum  value  in  about  three  hours. 

Radiothorium  is  the  most  powerful  and  convenient 
source  of  the  emanation  and  active  deposit  of 
thorium.  As  already  explained,  radiothorium  is  not 
separable  from  thorium  by  any  chemical  process. 
Freshly  prepared  thorium  compounds  contain  practi- 
cally all  the  radiothorium  of  the  original  mineral, 
but  its  parent  mesothorium  being  absent,  this  radio- 
thorium  in  the  course  of  a  few  years  decays.  Before 
it  decays  completely,  however,  mesothorium  has 
been  regenerated  by  the  thorium,  and  in  time  begins 
to  produce  fresh  radiothorium.  The  consequence 
is  that  commercial  thorium  compounds  always 
contain  more  or  less  radiothorium,  and  always, 
therefore,  furnish  more  or  less  of  the  emanation  and 
active  deposit.  But  the  amount  is  insignificant 
compared  with  what  can  now  be  obtained  from  a 
commercial  radiothorium  preparation.  Mesothorium, 
after  it  is  separated  from  the  mineral  and  left  to 
itself,  produces,  as  we  have  seen,  radiothorium. 


268  RADIOTHORIUM 

After  a  year  or  more  of  accumulation  these  two 
substances  may  with  advantage  be  separated.  A 
trace  of  a  thorium  salt  is  added  to  the  solution,  and 
then  precipitated  by  adding  ammonia  as  thorium 
hydroxide,  which  carries  with  it  the  whole  of  the 
radiothorium,  leaving  the  mesothorium  in  solution. 
This  radiothorium  preparation  in  turn  generates 
thorium  X,  and  after  a  few  weeks  becomes  a 
powerful  source  of  the  thorium  emanation  during 
the  few  years  it  lasts. 

It  is  of  interest  to  note  that,  were  it  not  for  the 
existence  of  mesothorium  intermediate  between  and 
chemically  distinct  from  thorium  and  radiothorium, 
the  separate  existence  of  the  latter  might  not  have 
been  suspected.  In  the  case  of  uranium  I  and 
uranium  II  there  is  no  such  intermediate  member, 
and  in  consequence  the  evidence  for  the  existence 
of  two  elements  remains  indirect.  One  can  hardly 
help  wondering  how  many  of  the  well-known 
common  so-called  elements  may  not  be  mixtures 
of  more  than  one  element  with  chemically  identical 
properties. 

Radiothorium  may  be  used  to  show,  in  a  striking 
way,  by  means  of  phosphorescent  screens,  many  of 
the  older  classical  experiments  on  the  growth  and 
decay  of  radioactive  substances  on  which  the  existing 
theory  of  atomic  disintegration  has  been  built  up. 
For  example,  if  a  radiothorium  preparation  or  old 


RADIOTHORIUM  269 

mesothorium  preparation  containing  radiothorium, 
is  kept  in  a  tube  through  which  a  puff  of  air  can  be 
sent  from  a  rubber  blower,  and  the  accumulated 
emanation  is  thus  blown  out  into  a  flask  internally 
coated  with  zinc  sulphide,  as  shown  in  Figs  8  and  9, 
it  will  cause  it  to  phosphoresce  brilliantly  in  the 
dark.  The  decay  of  the  emanation  in  the  flask  can 
then  be  watched  from  minute  to  minute  with  the 
eyes,  and  its  concomitant  reproduction  in  the  radio- 
thorium  tube  can  easily  be  demonstrated.  For 
example,  the  radiothorium  tube  may  first  be 
thoroughly  blown  out,  and  then  the  effect  observed 
of  blowing  through  it  Into  a  zinc  sulphide  flask 
immediately,  before  any  emanation  has  had  time  to 
accumulate,  and  then  after  waiting  successive 
periods  of,  say,  ten,  twenty,  thirty  seconds,  one,  two, 
ten,  or  more  minutes.  For  the  shorter  intervals  the 
amount  of  emanation  produced  is  very  nearly 
proportional  to  the  time,  but  for  the  longer  ones 
the  decay  of  that  produced  first  during  the  period  ot 
accumulation  begins  to  tell,  and  the  increase  with 
time  gets  less  and  less  So  that  after  five  or  ten 
minutes  no  more  results,  however  long  a  time  is 
allowed  to  elapse.  The  emanation  is  then  in 
"equilibrium,"  as  much  decaying  per  second  as  is 
produced  per  second. 

In  this  way  many  of  the  simple  laws  of  the  decay 
and  reproduction  of  the  emanation,  on  which  the 


270  THORIUM  A 

whole  superstructure  of  radioactive  theory  was  at 
first  largely  based,  may  now  be  shown  to  a  large 
audience.  All  this  original  work  was  done  with 
delicate  electrical  instruments  long  before  anyone 
had  ever  observed  a  single  visible  effect,  or  had  any 
other  than  indirect  electrical  evidence  of  the  exist- 
ence of  the  evanescent  emanation. 

The  most  recent  member  to  be  added  to  the 
thorium  disintegration  series  is  thorium  A,  the 
direct  product  of  the  emanation,  which,  on  account 
of  its  short  period  of  average  life — about  one-fifth 
of  a  second  only — had  hitherto  not  been  separately 
distinguished  from  the  emanation.  It  was  put  in 
evidence  by  Rutherford  and  his  colleagues  in  the 
following  ingenious  manner  :  An  endless  wire 
passed  along  the  axis  of  a  cylinder,  containing  a 
radiothorium  preparation,  through  holes  in  ebonite 
stoppers  closing  the  ends  of  the  cylinder,  and  over 
suitable  pulleys  outside  of  the  cylinder  driven  by  an 
electric  motor.  In  this  way  the  wire  was  kept 
passing  through  a  cylinder  filled  with  thorium 
emanation.  The  wire  was  connected  to  the  negative 
and  the  cylinder  to  the  positive  pole  of  a  battery,  so 
as  to  concentrate  the  active  deposit  on  the  wire. 
It  was  found  that  the  wire  immediately  after  leaving 
the  cylinder  was  intensely  active,  giving  out  power- 
ful a-rays,  and  capable  of  lighting  up  a  zinc  sulphide 
screen  brought  near  to  the  wire.  This  activity  on 


THORIUM  A  271 

the  wire  lasts  only  a  small  fraction  of  a  second,  so 
that  after  the  wire  has  moved  away  a  little  from 
the  cylinder  its  activity  has  practically  disappeared. 
Thus,  although  the  wire  is  being  driven  at  a  high 
speed  all  the  time,  it  is  only  the  part  immediately 
issuing  from  the  cylinder  which  is  active,  and  which 
causes  the  sulphide  screen  to  glow.  Thorium  A  is  a 
non-volatile  product  of  the  gaseous  emanation,  and 
is  attracted  to  the  negative  electrode.  But  almost 
as  soon  as  it  is  deposited  it  breaks  up,  giving  a-rays. 
On  the  principle  of  a  short  life  and  a  merry  one, 
the  effect  of  this  product  is  far  more  marked  than 
that  of  the  longer-lived  products  it  in  turn  produces. 
Though  it  would  be  easy  to  show  that  the  wire, 
after  the  large  activity  due  to  thorium  A  is  over, 
still  possesses  activity  due  to  the  products  formed, 
this  activity  is  usually  far  too  small  to  light  up 
a  phosphorescent  screen.  In  this  way  the  existence 
of  this  almost  hopelessly  unstable  element  has  been 
demonstrated.  There  are  many  problems  of  acute 
interest  in  connection  with  the  thorium  active  de- 
posit still  being  investigated,  but  their  consideration 
hardly  yet  comes  within  the  scope  of  this  volume. 

THE  ACTINIUM  DISINTEGRATION  SERIES. 

In  conclusion,  a  few  words  may  be  said  for  the 
sake  of  completeness  on  the  third  and  least  known 
disintegration  series,  but  one  which,  for  that  reason, 


272  THE  ACTINIUM 

is  the  most  interesting  to  the  student  of  the  present 
time.  In  addition  to  the  polonium  and  radium  sepa- 
rated from  pitchblende  by  M.  and  Mme.  Curie,  a 
colleague,  M.  Debierne,  was  successful  in  isolating 
a  third  new  radio-element,  to  which  he  gave  the  name 
Actinium.  So  far  as  is  known,  actinium  is  at  least 
a  fairly  permanent  radio-element,  for  although  it 
was  discovered  very  shortly  after  radium,  the 
original  preparations  have  retained  their  activity, 
and  no  certain  decay  with  lapse  of  time  has  taken 
place.  Actinium  is  separated  with  the  "  rare  earths  " 
in  uranium  minerals,  and  chemically  it  resembles 
most  closely  the  rare-earth  element,  lanthanum, 
although  it  is  probably  not  completely  identical 
with  it  in  properties.  In  radioactive  properties 
its  disintegration  series  is  very  closely  analogous  to 
that  of  thorium,  and  consists  of  eight  members,  in 
addition  to  itself,  the  first  of  which,  known  as  radio- 
actinium,  corresponds  with  radiothorium.  The  next 
is  actinium  X,  corresponding  perfectly  with  thorium 
X,  and  after  that  the  actinium  emanation,  actinium 
A,  B,  C,  and  D,  follow  in  regular  order,  almost 
exactly  as  in  the  thorium  series.  The  analogous 
products  in  the  two  series  in  each  case  give  out  the 
same  kinds  of  rays,  and  are,  so  far  as  is  known, 
chemically  identical  in  character.  But,  almost 
without  exception,  the  periods  in  the  actinium 
series  are  uniformly  shorter  than  in  the  thorium 


DISINTEGRATION  SERIES  273 

series,  the  longest,  that  of  radioactinium,  being  only 
twenty-eight  days,  and  the  shortest,  that  of  actinium 
A,  being  only  ^otn  °f  a  second.  The  full  series  is 
shown  in  Fig.  33. 


Actinium        Radio-  Actinium  Emanation.  Actinium. 

(?).            actinium.  X. 

28'  i  15  5  '6  0-003 

days.  days.  seconds.  second. 

0-^-0-5 

Actinium  Actinium  Actinium     Actinium 

B.  C.  D.  E. 

52-1  3'i  6-83      (unknown). 

minutes.  minutes.  minutes. 

FIG.   33. 

Whereas  it  is  customary  to  regard  the  uranium 
and  thorium  series  each  as  starting  from  a  primary 
parent  of  so  long  life,  that,  old  as  the  world  is,  some 
still  survives  unchanged,  the  problems  connected 
with  the  real  nature  and  origin  of  actinium  are  still 
shrouded  in  mystery.  It  is  possible  that  actinium 
may  be  an  independent  primary  radio-element,  like 
the  other  two,  but  this  has  not  been  the  view  that 
has  so  far  gained  most  support.  So  far  as  knowledge 
has  been  gained,  actinium  appears  to  be  found  only 
in  the  uranium  minerals  and  in  all  of  these  which 

have  been  examined  for  it.    It  is  natural  to  conclude 
T 


274  ACTINIUM 

from  this  that  it  is  a  product  of  uranium.  But  here 
a  difficulty  arises.  In  the  disintegration  of  actinium 
at  least  five  a-particles  are  given  out  per  atom  dis- 
integrating, representing  a  loss  in  atomic  weight 
of  20  units.  There  is  certainly  no  room  for  the 
actinium  series  between  uranium  and  polonium,  and 
there  is  no  evidence  that  it  comes  after  polonium. 

The  important  piece  of  evidence,  however,  which 
shows  conclusively  that  actinium  cannot  be  in  the 
main  uranium-polonium  series,  and  which  at  the  same 
time  serves  to  distinguish  this  series  from  the  others, 
and  to  make  it  practically  the  most  difficult  to  investi- 
gate, is  the  extraordinarily  small  relative  quantity  of 
actinium  in  uranium  minerals.  Although  the  actinium 
series  gives  out  at  least  five  a-particles  per  atom  as 
compared  with  eight  given  out  by  the  uranium- 
radium-polonium  series,  the  a-radiation  contributed 
by  the  whole  actinium  series  is  only  about  one- 
fifteenth  or  one-sixteenth  of  that  contributed  by  the 
uranium  series.  Whereas,  if  actinium  were  in  the 
main  line  of  descent  from  uranium,  the  a-activity 
of  its  series  should  be  of  the  order  of  five-eighths 
of  that  of  the  uranium  series,  in  accordance  with 
the  principle  discussed  on  p.  212.  Two  possi- 
bilities may  be  advanced.  Either  actinium  is  an 
entirely  separate  and  independent  primary  radio- 
element,  and,  if  so,  its  occurrence  always  in  uranium 
minerals,  and  only  in  those  minerals,  is  difficult  to 


MULTIPLE  DISINTEGRATION  275 

understand ;  or  actinium  may  be  derived  from 
uranium  as  a  branch,  or  offshoot,  not  in  the  main  line 
of  descent.  One  may  suppose  that  at  some  stage  of 
the  disintegration  of  the  uranium  atom  a  choice  of 
two  modes  of  disintegration  presents  itself.  The 
large  majority  of  the  atoms  choose  one  way — the  way 
leading  to  polonium — whilst  a  small  minority  choose 
a  second  way,  the  way  leading  through  the  actinium 
series.  If  this  is  true,  it  can  be  calculated  that  out  of 
every  eight  uranium  atoms,  seven  go  through  the 
main  line  of  descent  towards  polonium,  and  one  goes 
through  the  actinium  line.  This  mode  of  explaining 
actinium  has  been  rendered  the  more  worthy  of 
credence  because  of  the  discovery  very  recently  of 
an  actual  case  of  such  a  multiple  disintegration  in  the 
end  of  the  thorium  series,  among  the  active  deposit 
products,  which,  however,  cannot  be  dealt  with  here. 
The  practical  aspect  of  this  peculiarity  of  actinium, 
is  that  the  substance  is  very  much  rarer  and  more 
difficult  to  obtain  than  the  members  of  either  of  the 
other  two  series.  If  it  were  more  common,  it  would 
lend  itself  to  many  demonstrations  and  experiments 
similar  to  those  detailed  under  radiothorium,  but  of  an 
even  more  striking  character.  Actinium  is  relatively 
poor  in  penetrating  rays,  and  even  the  most  active 
preparations  it  is  possible  to  procure  are  disappoint- 
ing in  this  respect  when  compared  with  radium. 
The  glory  of  actinium,  however,  is  its  emanation,  a 


276  ACTINIUM  EMANATION 

gaseous  disintegration  product,  precisely  analogous 
in  every  respect  to  those  of  radium  and  of  thorium, 
but  having  a  period  of  average  life  of  only  5  *6  seconds. 
The  usual  principle  of  a  short  life  and  a  merry  one 
applies.  The  dominating  characteristic  of  the  radio- 
activity of  actinium  preparations  is  the  emanation 
that  is  given  off.  In  the  dark  room,  if  a  preparation 
is  held  over  a  zinc  sulphide  screen,  the  emanation 
diffusing  away  lights  up  the  screen  in  patches,  which 
are  wafted  from  one  part  of  the  screen  to  another  by 
draughts  or  any  gentle  puffs  of  air.  The  rapid 
decay  of  the  emanation  and  corresponding  rapid  re- 
generation from  the  actinium  preparation  makes  it 
quite  possible  to  experiment  thus  with  the  emanation 
in  the  open-air  of  the  room.  Whereas  if  the  radium 
emanation  were  dealt  with  in  this  way,  once  it  had 
been  dissipated  throughout  the  air  of  a  room,  some 
weeks  would  have  to  elapse  before  a  fresh  supply 
was  available.  Giesel,  who  rediscovered  the  sub- 
stance subsequently  independently  of  Debierne, 
actually  named  it  "  Emanium  "  before  it  was  found 
to  be  identical  with  actinium. 

The  only  other  product  of  actinium  which  calls 
for  special  mention  is  actinium  A,  the  direct  pro- 
duct of  the  emanation,  which,  like  thorium  A,  has 
an  extraordinarily  short  period  of  life.  Indeed, 
actinium  A  is  the  most  unstable  element  known, 
its  period  being  only  about  ^oth  of  a  second.  It 


ACTINIUM  A  277 

may  be  put  into  evidence  by  the  same  device  as  that 
described  for  thorium  A  (p.  270),  but,  of  course, 
the  endless  wire  has  to  be  driven  considerably  more 
rapidly  than  is  necessary  to  exhibit  the  thorium 
product.  As  a  matter  of  fact,  a  forgotten  experi- 
ment of  Giesel,  eight  years  before  the  discovery 
of  actinium  A,  clearly  puts  the  existence  of  that 
substance  into  evidence  when  rightly  interpreted. 
If  a  zinc  sulphide  screen  is  brought  opposite  to 
the  open  end  of  a  tube  containing  an  actinium 
preparation,  and  a  little  distance  away  from  it,  there 
is  a  diffuse  luminosity  produced  on  the  screen  in  the 
dark,  due  to  the  emanation  escaping  from  the  tube. 
If  the  screen  is  now  connected  with  the  negative 
pole  of  an  electrical  machine,  instantly  there  flashes 
out  on  the  screen  a  sharply-defined  bright  spot  of 
the  same  geometrical  form  as  the  opening  of  the 
tube.  On  discharging  the  screen  this  spot  instantly 
disappears.  Giesel  thought  he  was  dealing  with  a 
new  kind  of  radiation,  attracted  by  a  negatively 
charged  surface,  and  called  the  supposed  radiation 
the  "  E-ray,"  in  brief  for  "  emanation  ray."  How- 
ever, it  is  not  the  ray,  but  the  excessively  short- 
lived product  giving  an  ordinary  a-ray,  which  is 
attracted  to  the  negative  surface ;  but  owing  to  the 
infinitesimal  time  this  product  remains  in  existence 
it  appears  as  if  it  is  the  ray,  rather  than  the  product, 
which  is  affected  by  the  electric  field.  Another  way 


278  THE    UNSOLVED  PROBLEM 

of  showing  the  same  experiment  is  to  coat  a  wire 
with  zinc  sulphide,  and  to  immerse  it  in  a  flask  con- 
taining an  actinium  preparation.  On  charging  the 
wire  negatively  to  the  flask,  the  zinc  sulphide  instantly 
flashes  out  and  remains  brilliantly  luminous  ;  but 
on  discharging  the  wire,  the  luminosity  disappears 
apparently  instantaneously. 

Thus,  with  the  discovery  of  this  last  substance, 
the  science  of  radioactivity  now  embraces  over 
thirty  examples  of  elements  in  the  course  of  spon- 
taneous change,  with  periods  varying  from  tens  of 
thousands  of  millions  of  years  on  the  one  hand,  to  a 
few  thousandths  of  a  second  on  the  other,  whilst  the 
existence  of  other  products,  with  periods  of  the  order 
of  a  millionth  of  a  second,  have  been  indicated 
(p.  226).  It  is  unlikely  that  very  many  more  of  these 
unstable  elements  remain  to  be  discovered,  unless 
some  entirely  unknown  and  unsuspected  source  of 
radioactive  materials  is  found.  But  science  has  still 
much  to  glean  from  the  examples  already  known  of 
that  most  fundamental  and  inaccessible  problem, 
which  at  the  same  time  appears  to  be  the  problem  of 
ultimately  the  most  practical  significance — the  real 
internal  nature  of  matter.  How  is  the  atom  of 
matter  put  together,  and  how  can  it  be  pulled  apart  ? 
These  are  the  questions  which  the  discoveries  of 
radioactivity  raise  in  a  pressing  form  without  as  yet 
affording  a  hint  of  the  answer. 


INDEX 


ar-pairticles,      Collision     of,     witb 
matter..  :So~9 

—  Cocuaectiott  of,  with  helium,  6-2, 

$4,  t2>-42 

—  Energy  of,  £5 

—  froci  radium  itself,  129 

—  from  the  emanation.  i  to,.  205 
-  Individual,  5$,  62-5,  S6 

—  IJmrting  velocity  of,  §9.,  91 

—  Miss  of,  $>,  1  56 

—  Number  of,  expelled  by  radium. 

57,  59,  62-5 

—  Positive  charges  carried  by, 

$5.  :£> 

—  Proof  of  identity  trith  beiium 


Scattering  of,  $9 
Velocity  of,  $5, 


150,  224 


bsorption  of,  46 

by  air.  49-50 

Connection  between  range  of, 
and    period    of    substance, 

2C26 

Magnetk  defection  of.  $5,  $4 
Making  patbs  of  ^-tabi-e,  90-1 
Range  of,  49>  e>5>  lS7 


of 


151-5, 


Acttniinn,   disintegration    series, 
256,  27i-S 

—  Emanation,  27-2,  275 
-  Origin  <rf,  275-5 

—  Production  of    beiium   from. 


Actinium  A,  275,  276-^ 
—  B.  C,  Atid  IX  272 

Actix-^e  deposit  of  actinium,  272-$ 
--  of  thorium,  207-71 
--  of  radium.  191-202 
---  Residual  activity  from, 


Age  of  the  earth.  55,  105,  244-52 
Ages.  The-  geological  and  incan- 

descent. 246-7 

Alkaline-earth  elements,  117.  145 
Alternative    theories     of    radio- 

active energy.  94.  125 
Analogies    between    the    disinte- 

gration serieSv  259-62.  27-2-5 
Argon.  1  16.  117.  154 
Atom.  Peiirution  of.  145-52 
Atomic  disintegration.  54.  $2.  9?, 
124.  150,  155.  155,216,  271 
--  Cause  of.  159 
--  Multiple.  275 

—  property,  Radioactivity  an,  17, 

IS,  21.  94,  102.   H5/I55 

—  synthesis,  24$,  27$ 

Atoms,  5,  16,  57.  65,  $4,  SS,  nS, 
146.  219-25 

—  Interpenetration  of.  $£ 
Average  life.  Determination  of  ,  i6i 
--  of  common  elements,  217 
--  of  emanation,  157 

--  of  radium.  165.  174 
--  of  uranium,  162,  174 
--  Period  of.  157 


^-particles,  70-^5 

—  Charge  of,  70,  72,  $2 


279 


280 


INDEX 


/3-particles,  Mass  of,  80 
-  Velocity  of,  80- 1 
/3-rays,  39-92 

—  Magnetic  deflection  of,  66,  82 

—  Making  paths  of  visible,  90-1 
Barium,  21,  115,  187 
Becquerel,  Henri,  8,  9,  10,  179 
Bismuth,  21,  187,  207,  262 
BoUwood,  B.,  174,  187 

Bragg,  Professor,  48,  49,  63,  87, 

89 
Bunsen,  104 

C 
7-rays,  43-45 

—  Radiograph  by,  44 
Caesium,  104 

Calcium,  Absorption  of  gases  by, 

74.  139 

Cascade  of  changes,  102 
Cathode-rays,  73-82 
Cause    of   atomic    disintegration, 

159 

Chance  of  disintegration,  155 
Change,  Law  of  radioactive,  156 

—  of  radio-elements,  97,  103, 125, 

127  et  seq. 

Chemists  and  radioactivity,  152 
Cloud  method  of  making  paths  of 

rays  visible,  90-1 
Conservation  of  radioactivity,  122 
Constancy  of  radioactivity,  14,  17, 

32,  33»  37»  6l»  95.  96,  107,  126, 

237 
Control  of  natural  energy,  8,  17, 

238,  252,  256 
Corpuscular  theory  of  radiation, 

54.  56-65 
Cosmical  aspect  of  life,  247-8 

—  energy,  33,  167,  240-8,  246 
Cost   of   scientific   investigations, 

26,  27 
Crookes,  Sir  William,  20,  59,  73, 

80,  179 

Crookes'  tubes,  73,  81 
Curie,  M.  and  Mme.,  16,  18,  21, 

104,  115,  173,  191,  205,  272 

D 

"  D3  "  line,  134,  138,  140 
Dalton,  John,  147,  150 


Debierne,  137,  272,  275 
Decay  of  radioactivity,  96,  121 
Definition  of  the  atom,  145-52 
Detection  of  infinitesimal  quan- 
tities, 104,  106,  114,  118,  124, 
126,  130,  152 

Determination  of  average  life,  162 
Dewar,  Sir  James,  74 
Discovery  of  radioactivity,  8 
Discrete  theory  of  radium  rays, 

56,  62 

Disintegration,    see    Atomic    dis- 
integration 
— ,  Chance  of,  155 

—  series,      Analogies      between, 

259-62,  272-3 

of  actinium,  256,  271-8 

of  thorium,  245,  256-71 

of  uranium,  168-209 

Doctrine  of  energy,  26,  38,  51,  93. 
246,  254 

E 

"  E-ray,"  277 
Earth,  Age  of  the,  35,  103,  244-52 

—  Internal  heat  of,  245-6 
Effects  of  radioactivity,  11-13 
Electric  current,  Action  of  magnet 

on,  70 
Electricity,  Discharge  of,  n,  18, 

25,  47,  64,  90 
Electro-magnet,  66 
Electron  theory  of  matter,  151 
Electrons,  77-82,  88,  151 
Electroscope,  Gold  leaf,  n,  23,  59, 

116 

Elements,    Stability  of,   99,   217, 
228-9 

—  Unchanging  character  of,  99, 

101,  228-9 
Elixir  of  life,  250 
Emanation  of  radium,  107-30 

—  a-particles  from,  no,  203 
-  Atomic  weight  of,  118,  142 

—  Average  life  of,  157,  161,  170 

—  Chemical  nature  of,  116,  117, 

145 

—  Condensation  of,  111-4 

—  Density  of,  118 

—  Heat  generated  by,  118,  119, 

233 


INDEX 


281 


Emanation,  Physiological  action 
of,  114 

—  Rate  of  decay  of,  121,  123 

-  Reproduction  of ,  122,  123,  125, 

170 

-  Spectrum  of,  117 

-  Volume  of,  114,  165 

-  of  actinium,  272,  275 

-  of  thorium,  191,  261,  266-71 
Emanations  and  radiations  con- 
trasted, 107 

Emanium,  275 

Energy,  Internal,  of  matter,  94, 

97,  98-101,    120,    121,    126, 

132,  149,  232-42 

—  Measurement  of,  30,  98 

-  of  coal,  30,  31,  97,  167 

-  of   radioactive  substances,   4, 

8,  14,  81,  87,  93,  126,  127, 
236 

-  of  radium,  31,  118,  167,  236 

-  of  uranium,  233-6 

-  Transformers  of,  94,  95,  125 
Ephemeral  transition-forms,  102, 

127,     160-6,     169,      180,     271, 

276-8 
Equilibrium,     Radioactive,     125, 

131,  163,  269 
Ether,  The,  52,  53,  78 
Evolution  of  elements,  189,  225, 
228 

—  of  universe,  36,  167,  241-7 
Existence,  Struggle  for,  8,  250-5 


Facts  and  theories  of  radio- 
activity, 124,  150 

Faraday,  66,  77 

Fluorescence,  9,  24,  43,  74,  91, 
108,  109,  268 


Gas,  A  radioactive,  107,  in,  115 

Geiger,  63 

Geological  bearing  of  radioac- 
tivity, 36,  103,  241-8 

Geology,  Controversy  between 
physics  and,  36 

Giescl,  26,  137,  275,  277 

Gold  currency,  216 


H 

Hahn,  Otto,  258 

Heat  generated  by  radium,  12,  25, 
31,  118,  166,  245 

in  the  earth,  245-6 

Helium,  62,  84,  116,  130-42 

-  Discovery  of,  133 

-  Liquefaction  of,  134 

-  Prediction      concerning      the 

origin  of,  135 

-  Production  of,  by  radium,  130, 

136 
-  by  actinium,  137,  140 

by  thorium  and  uranium, 

138-40 

—  in  radioactive  minerals,  133,135 
Volume  of,  135 

—  Spectrum  of,  137 
Herschell,  Sir  John,  221,  225 
High  vacua,  72,  74 
Huggins,  Sir  William,  149 


Incandescent  age,  246-7 

—  gas-mantle,  19,  256 

Increase  of  activity  of  radium 
with  time,  22,  214 

Indifference  of  radium  to  its  en- 
vironment, 37,  107,  225 

"  Induced  radioactivity,"  191 

Inertia,  79 

Infinitesimal  quantities,  Detection 
of,  104,  106,  114,  118,  124,  126, 
130,  152 

Inglis,  J.  K.  H.,  157 

Intermediate  substances,  102,  105, 
106,  184-9 

Internal  energy  of  matter,  94,  97, 
98-101,  120,  121,  232 

-  heat  of  earth,  245-6 
Interpenetration  of  atoms,  88 
lonisation,  88,  90 

Ionium,  187,  212,  213,  226,  260 

-  Estimated  period  of,  188,  226 
—  and   uranium    X,    Connection 

between,  188 


Joachimsthal  mine,  20,  27,  210 
Joly,  Professor,  243-6 


282 


INDEX 


K 

Kelvin,  Lord,  28,  52,  245 
Kirchoff,  104 

L 

Lanthanum,  272 

Law  of  proportionality,  164*  172, 
210-2 

—  of  radioactive  change,  155 
Lead  and  radium,  Connection  be- 
tween, 21,  105,  208,  209 

Life  from  the  cosmical  standpoint, 
247-8 

—  of  radio-elements,  127 

—  Period  of  average,  157 
Light,  Nature  of,  51,  55 

—  Velocity  of,  53,  81 
Limitations  of  knowledge,   5,   8, 

92,  238,  246-8 

M 

Macdonald  laboratories  of  M'Gill 
University,  124 

Mackenzie,  T.  £>.,  182 

Magnetic  deflection  of  cathode- 
rays,  76 

Maintenance  of  radium,  168-88 

—  sun's  energy,  33,  167,  246 
Mayckwald,   Professor,    205,    206, 

264 

Mass  of  the  electron,  77-80 
Matter,  Electron  theory  of,  151 

—  Unsolved  problems  of,  151,  278 
Maxwell,  J.  Clerk,  220,  225,  249 
McCoy,  H.  N.,  174 
Measurement  of  energy,  30,  98 
Mental  pictures,  152 

Mercury,  118,  208 

Mesothorium,  258-66 

Minerals,  Helium   in  radioactive, 

133.  135 

—  Lead  in  radioactive,  21,  208 

—  Quantity    of   radium    in,    22, 

104,  172-8,  210 

—  Ratio    between    quantities  of 

uranium  and  its  products  in, 

211 

Minimum  quantity  of  helium  de- 
tectable, 140 
radium  detectable,  23,  59 


Molecules,  3,  150,  220 
Monazite  sand,  257,  265 
Monopoly,  Radium  a  scientific,  27 

N 
Negative  and  positive  electricity, 

7i 

Neon,  116 
Newton,  53,  54 
"  Niton,"  108 
Nomenclature    concerning   atoms 

and  molecules,  147-50 
Non  -  separable    radio  -  elements, 

212,  257,  268 


Onnes,  K.,  134 
Ouroboros,  249 


Parent  of  ionium,  188 

—  of  radium,  170-189 
Penetration  test  of  rays,  10,  41, 

42,  43,  no 
Period  of  average  life,  157 

connection  with  range 

of  a-rays,  226 

—  half  change,  160 
Periodic  law,  146 
Perpetual  motion,  29,  33,  82 
Phosphorescence,      see      Fluores- 
cence 

Photographic     effects     of    radio- 
activity, n,  19,  25,  91,  in 
Physical  impossibility,  38 
Pitchblende,    20,    104,    177,    213, 

258 

Platino-cyanides,  24,  50,  178 
Polonium,  21,  62,  187,  204-7,  213, 

274 
Positive  and  negative  electricity, 

7i 

Prediction  of  origin  of  helium,  135 
Proportionality,  Law  of,  164,  172, 

210-2 

Q 

Quantity  of  helium  detectable  by 
spectroscope,  140 


INDEX 


283 


Quantity  of  helium  in  minerals, 

135 

—  of    radium    in    minerals,    21, 

104,  172-8 

R 

Radiant  matter,  73,  80 
Radiation,  Nature  of,  51-6 
Radiograph  by  7-rays,  44 
Radio-tellurium,  205,  207 
Radio-thorium,  258-68 
Radium  and  uranium,  connection 
between,  173-88 

-  Average  life  of,  165,  174 

—  Chemical  nature  of,  21 

—  clock,  82 

-  Maintenance  of,  168-88 

-  Substitute  for,  214,  258,  265 
Radium    A,    123,    146,    194-202, 

2II-2 

-  B,  123,  146,  194-202 

-  C,  144,  194-202,  211-2,  226 

—  D,  212-3,  258,  261 

—  D,  E,  and  F,  204-7 

-  F,  Identity  of,  with  polonium, 

206-7 
Ramsay,   Sir   William,    108,    114, 

116,  118,  134,  136,  165,  258 
Ratio  between  uranium  and  its 

products,  211 
Rayleigh,  Lord,  116 
Rays   of  radioactive   substances, 

12,  39  et  seq. 

Recoil,  Radioactive,  143-4 
Recovery     of     radioactivity     of 

radium,  107,  121 
Rowland,  Professor,  222 
Royds,  T.,  142 
Rutherford,  Professor,  41,  42,  63, 

64,  83,  107,  in,  119,  124,  136, 

142,  166,  174,  190,  258,  270 


Scattering  of  a-particles,  89 
Schuster,  Professor,  223 
Sidot's  hexagonal  blende,  50 
Silk  tassel  experiment,  25,  47 
Simplon  Tunnel,  Radium  in  rocks 
of,  244 


Spectroscope,  104,  126,  127,  133, 
136..  140,  182 

Spinthariscope,  59,  63 

Stability  of  elements,  99,  217, 228-9 

Standard,  The  International 
radium,  23 

Struggle  for  existence,  8,  250-5 

Strutt,  82,  174,  242-3 

Substitute  for  radium,  214,  258, 
265 

Successive  changes  of  radio  ele- 
ments, 102,  106,  123,  153,  162, 
180-8,  194,  204-9 

Sun's  energy,  Maintenance  of,  33, 
167,  245-7 

Synthesis  of  atoms,  248,  278 


Tait,  5,  28,  34,  36 
Thallium,  208 

Theories     and     facts     of     radio- 
activity, 124,  150 
Thomson,  Sir  J.  /.,  80 
Thorium,  18,  129,  134,  135,  138- 
40,  187,  191,  213,  256-71 

—  Active  deposit  of,  267-71 

—  disintegration  series,  245,  256- 

7i 

—  Production   of    helium   from, 

138-40 

—  Ultimate  product  of,  262 
Thorium  A,  261,  270-1 

—  B,  C,  D,  226,  261 

—  Emanation,  261,  266-71 

—  X,  261,  268 

Total  energy  in  radium,  166 

in  uranium,  234,  236 

Transcendental  character  of  radio- 
activity, 37,  8 1 

Transformers  of  energy,  94,  95 
Transmutation,    18,   97,   98,    100, 
237-40,  250 

U 

Ultimate  product  of  thorium,  262 
-  products  of  radium,  105,  131, 

171,  207,  208 

Ultra-material  velocities,  88 
Unchanging  character  of  elements, 
99,  101,  228-9 


284 


INDEX 


Unsolved  problem  of  matter,  151, 

278 
Uranium,   9,    17,    134,    135,    162, 

173-89,    208-9,    233-7,    259, 

260,  266 

—  Average  life  of,  162,  174 

—  Production   of    helium   from, 

138-40 

—  and    radium,  Connection    be- 

tween, 173-88 

Uranium  I  and  II,  180,  212,  226, 
260,  268 

—  X,  179-82,  188,  259 

and     ionium,     Connection 

between,  188 


Vacua,  High,  72,  74 
Value,  Gold,  physical  explanation 
to  account  for  the  unchang- 
ing, 217 

-  of  radium,  25,  26,  215 
Velocities,  Ultra-material,  88 


Velocity  of  cathode-ray  particle, 
81 

—  of  light,  53,  8 1 

Visible,  Making  the  paths  of  rays, 

90-1 
Volume  of  helium  in  minerals,  135 

—  emanation  in  equilibrium  with 

radium,  114,  165 

W 

Wave  theory  of  light,  55 
Whytlaw-Gmy,  R.,  118 
Willemite,  50,  74,  76,   108,   109, 

112 

Wilson,  C.  T.  R.,  89 
Writing  by  radium,  25 

X 

X-rays,  9,  42,  44,  53,  109 

Z 

Zinc  sulphide,  50,  no,  197,  269, 
271,  275,  278 


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